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The Materials

This chapter covers the general properties of stone, such as hardness, toughness, and resistance to weathering, and discusses the particular properties of the stones commonly available for carving.

Hardness and Toughness

What we informally call "hardness," scientists and engineers recognize as a group of distinct but related properties that may or may not occur together in similar proportion. For instance, a material can be very hard, in the sense of being able to scratch other materials, yet have little ability to withstand an impact.

Think of two "hard" materials: oak and a soft sandstone. You might be able to gouge into the sandstone with a steel tool using only hand pressure, but be completely unable to penetrate the oak by the same means. On the other hand, a saw that could easily cut through the oak, would be quickly destroyed by the sandstone. Moreover, the chunk of sandstone could be used to grind away oak or steel, yet the same stone could be pulverized by hitting it with the oak block.

The sandstone is more abrasive than the oak, and the oak is tougher than the sandstone. So which is harder? It depends on what scale you use. The Mohs scale is the most common ranking of the relative hardness of minerals, ordering them in terms of which can scratch the other. If stone X can scratch stone Y, then X has a higher Mohs number. Scratching is the most useful criterion for stone, but there are many other tests for other materials. For instance, metal hardness is often measured by the Rockwell test, which measures how deeply a sample is dented by a standard impact.

While no single number captures all of a stone's properties, Mohs hardness captures a lot of what we care about in stone, so it is a good point of departure for describing the physical properties of a stone.

Talc, the softest solid mineral, is assigned the index 1 on the Mohs scale; you can scratch it with your fingernail. Diamond, which is scratched by no natural mineral except other diamonds, is assigned a hardness index of 10. Minerals are given fractional numbers if they will scratch one of the Mohs minerals but are themselves scratched by the next. The scale is purely ordinal, meaning that the difference in the Mohs numbers for two materials is not necessarily proportional to the difference in their hardness, but only reflects their ordering. All that the numbers tell you is that the material with the greater number is harder, but you can't infer how much harder. For example, corundum (9), is considered to be twice as hard as topaz (8), but diamond (10) is considered to be four times as hard as corundum. Not every sample of a given minerals will have the exact same hardness. Even diamonds come in a range of hardnesses--the hardest varieties are prized for their ability to cut and polish the rest, something no other mineral can do.

The table below gives the reference mineral, familiar materials that can scratch them, and sculpted stones of comparable hardness. Anything material in the "scratched-by" column will scratch all materials with a lower Mohs number.

Mohs Substance Scratched By Carving Stone
1 Talc Fingernail None
2 Gypsum 24-karat gold Alabaster, soapstone
3 Calcite Copper Limestone, soft marble
4 Fluorite Steel knife Typical marble
5 Apatite(tooth enamel) Granite Hard marbles
6 Orthoclase Feldspar Silica glass Softer granites
7 Quartz Tungsten carbide Granite, diabase
8 Topaz Sapphire, ruby None
9 Corundum (ruby) Silicon carbide None
10 Diamond Diamond None

In English, the chart above tells you that stones softer than 5.0 or less can be worked with steel tools, that all carvable stones can be worked with carbide tools or silicon carbide abrasives, and that silicon carbide will cut anything except diamonds. The chart does not say it, but in practice, stones with a hardness of around 2.0 can be worked with knives and hand saws made for woodworking. Stones harder than 2.5 will quickly damage any tool with knife-like tool edges, so wider angle bevels and with a harder temper are necessary.

Most sources still report what this chart suggests, that industrial silicon carbide is second only to diamond as the hardest substance, and this was true until relatively recently. However, cubic boron nitride, also called cBN, another synthetic material, is significantly harder than silicon carbide, and is available in the form of grinding wheels and burrs. Tools made of cBN are primarily for high-tech machining and are not hardware-store items, so for most artist's purposes, silicon carbide and diamond are the two hardest abrasives.

Aluminum oxide-based abrasives will barely scratch granite and diabase, but cut marble easily. While excellent for grinding and sharpening steel tools, they will not work on tungsten carbide. Silicon carbide, often sold under the trade name Carborundum, is the abrasive found in green grinding wheels, and in many whet stones. It is used for shaping and polishing hard stones, as well as for sharpening carbide tools. Silicon carbide grinding wheels can be dressed and re-shaped with diamond tipped tools.

Both aluminum oxide and silicon carbide are usually bonded into a material that looks like stone, and are called "stones". The various abrasive stones are color coded, in addition to having their constituent abrasives and several other propertes encoded on the labels. For most purposes, it is enough to know that white, pink, blue, and gray are usually aluminum oxide, and can be used on most metals as well as on the softer stones. Green stones and dark gray stones are usually silicon carbide, which can be used on all stone, and steel, as well as on tungsten carbide tools. Diamonds tools usually look like plain metal with a slightly rough surface. They will grind anything, but the are not usually either necessary or well-suited for grinding sculptural stone. The are excellent, however, for circular saw blades, and will easily cut granite.

Though diamond is by far the hardest natural material, it is not very tough--you can pound a diamond into powder with a soft brass hammer. In fact, before the historically recent invention of techniques for sawing and grinding diamonds using diamond dust, diamonds were traditionally cut by cleaving, i.e., splitting on natural planes, using steel chisels. Thus, for processes that use impact on hard materials, rather than scraping, the best choice is tungsten carbide.

The difference between hardness and toughness is illustrated well by jadeite, which has a Mohs hardness number similar to that of granite, but is so tough that it was used for hammerheads by ancient stone workers, and can even be used to hammer steel tools.

Lines-of-cleavage and other anisotropies (physical properties that vary with orientation) are common features of many kinds of stone, especially of sedimentary stones such as limestones, and metamorphic stones derived from sedimentary origins, such as the marbles. For instance, such stones will often split relatively easily along the lines of their "bedding", i.e., the planes in which the original sediments were laid down. Even within a seemingly uniform block of marble, a carver will sometimes find that the same tool works differently from one side of the block than the other.


Exposure to the elements attacks stone in many ways, and every kind of stone is vulnerable to weather some degree, but sensitivity varies. On a human time scale, most granite and similar stones are more or less imperishable, whereas other stones might show the effects of weathering in a single year, and be substantially destroyed in less than a century. A partial list of the ways in which stone is attacked by the elements includes:

  • Water that seeps into porous or cracked stone expands as it freezes, splitting the stone or crushing the fine structure so that it crumbles. Stone may appear to be untouched for many years, the abruptly fall apart in a year or so, as the damage reaches the point where the openings enlarge to some critical size.
  • Ordinary rain will simply wash away some stones, particularly limestone. Acid rain rots and erodes all carbonate stones much more quickly.
  • Other kinds of precipitation, e.g., snow and dew, can have the same effect as rain.
  • Biological effects: Bird droppings attack many kinds of stone chemically. Lichen, moss, and other plants can inhabit cracks and pores in the stone, causing various kinds of damage, both chemical and mechanical.
  • Extreme changes in temperature stress stone mechanically, e.g., the hot sun expanding the surface relative to an ice-cold interior. Some very hard stones such as diabase may be quite fragile with respect to temperature.
  • Naturally-formed minerals or salts deposited by water can attack the surface chemically, or damage it mechanically due to the growth of crystals.


Some marbles and limestones weather better than others, but all such carbonate stones will eventually be eroded by exposure to rain water. Even rain of natural acidity attacks the surface of marble, slowly rendering it chalky and porous, or giving it a sugary, crumbly texture. In time, the surface rot penetrates the stone and ultimately destroys the deeper structure of the stone as well as the surface.

Limestone erodes more gracefully, tending to wash away slowly from the surface in. The image below shows the degree to which painted numbers have protected the surface of a limestone wall from erosion. The wall (in Park Slope, Brooklyn) was built around 1895, and the numbers were probably painted on much more recently, yet the painted surface is as much as 3/8 of an inch higher than the adjacent unpainted surface. The thickness of the paint itself is negligible; stone washed away by rain water constitutes almost all of the difference. Note that the lines of bedding (which happen to be horizontal) have been brought up as the stone washes away faster in some areas than in others. The right side of the same picture shows severe erosion on a carved piece of limestone. Note the emergence of tiny fossil shell fragments, and the exposure of the original sedimentary layers.

The effect of weathering is obvious on the majority of marble monuments and gravestones in North America that date from the second half of the 19th Century or earlier. Many are already in quite bad shape.

Left: limestone protected from erosion by painted lettering. Right: badly eroded limestone carving. Note shell fragments and sediment layers.


Acid rain accelerates this process. Water containing dissolved carbon dioxide is always acidic due to the temporary conversion of the carbon dioxide to carbonic acid. Almost all naturally occurring water has at least some carbon dioxide dissolved in it, because the atmosphere contains approximately 0.038 carbon dioxide.

Naturally occurring water is only slightly acidic because 0.038 is not a very high concentration. Soda water, on the other hand, is super-saturated with carbon dioxide. To see the sensitivity of marble to the acidity of rainwater in fast-motion, splash some fresh soda-water or cola stand on a clean piece of polished marble. Absent a protective coating such as wax or oil, even a brief exposure to fresh soda water will etch a matte-finished mark on the polished surface.

Rain more that is significantly more acidic than results from naturally dissolved carbon dioxide is known as "acid rain." The excess acidity results not from carbonic acid, but from sulphuric and nitric acids, usually derived from sulfur and nitrogen compounds that are injected into the atmosphere by burning fossil fuels. Volcanic eruptions can also produce acid rain. The deposition of the resulting acid may occur hundreds of miles from the point of origin, and need not be in the form of rain. Other kinds of precipitation such as snow, fog and dew, as well as direct deposition of acids in the absence of any visible precipitation, are also factors.

Acidity and alkalinity are specified by the pH scale. Pure distilled water is neutral, and has (by definition) a value of pH 7.0. Numbers between 0.0 and 7.0 indicate acidity, and numbers between 7.0 and 14.0 indicate alkalinity. Thus, ordinary water containing the normal concentration of atmospheric carbon dioxide is already slightly acidic with pH of 5.5. Rain with a pH less than 5.0 is considered to be acid rain. Very few aquatic animals can withstand a pH of less than 4.5 to 4.0 and many die if the acidity as low as 5.0. In extreme cases, acid rain can be more acid than vinegar, which is approximately 2.9.

The following table gives a few sample pH values for comparison.

Liquid pH Value
Battery acid 0.5
Lemon juice 2.4
Soda water 2.5
Vinegar 2.9
Acid rain < 5.0
Natural water 5.5
Distilled water 7.0
Soap 9.0-10.0
Pure lye (e.g., drain-cleaner) 13.5

Carvable Stones

Only the most common carvable stones are described here--there are many more. Within these generic categories stones may have a wide variety of properties. Color can vary widely--marble, for instance, can be anywhere from snow white to black, or brightly colored.

You may find other stones quarried locally, or have access to fieldstone or stone from exposed outcrops. It may be worth identifying such found stones and try out their carving properties. Some are good for carving and others are not, either because of poor workability, deep effects of weathering, or because they contain asbestos or other hazardous minerals such as toxic metals. (Serpentine is a good example. It's great to carve, but many varieties contain asbestos.) It's a good idea to have any stone not specifically sold for carving looked at by geologist or rock-hound before carving it.

Artificial materials like brick and concrete can aso be carved with the same techniques.

Carbonate Stones

The carbonate stones, of which limestone and marble are the most familiar, are composed primarily of calcium carbonate (CaCO3), which mostly originates as the shells or skeletons of sea-life. A thick ooze of this material covered much of the floors of ancient oceans just as it does today, accumulating to tremendous thickness and eventually slowly hardening into chalk or limestone under the weight of overlying deposits.

The limestones are thus sedimentary rocks, being composed of either the original shells themselves, or of calcium carbonate that has dissolved from such deposits and subsequently recipitated from solution. Other minerals, such as sand and clay, may be present in smaller amounts, either uniformly mixed or concentrated in layers. You'll find sand, pebbles and even sea-shells and other fossils in many varieties of limestone. Calcium magnesium carbonate (CaMg(CO3)2) is present in dolomite and related stones.

The marble family, which overlaps to some degree with the limestones, are metamorphic. Marble originates as limestone, but is subsequently transformed by heat and pressure, which give the calcium carbonate a denser crystal structure. Although the original limestone may have once contained fossils, because of the metamorphosis, marble generally does not, although it may occasionally contain inclusions of other minerals. While some so-called marbles, notably Belgian black marble, may be alternately classified as limestone, and marble, conversely, sometimes be defined simply as limestone capable of taking a high polish, to a geologist, the key distinction is sedimentary vs. metamorphic. (Belgian black marble is thus technically a limestone, not a marble.)

Chalk is chemically identical to limestone, and of the same origin, but is softer and less compressed. Marl is chalk or limestone with an admixture of clay and mud originating from the outflow of ancient rivers.

Some carbonate stones originated not directly from sea-shells, but from calcium carbonate laid down in other ways.

Tufa, for instance, is composed of calcium carbonate that was precipitated from fresh water lakes which for one reason or another have been fed by mineral-rich waters. Similar to limestones originating in ancient seas, tufa is usually softer than ordinary limestone, and often has the property of hardening upon exposure to air. Some tufas, when fresh, are soft enough to cut with a carpenter's saw, yet become quite hard after a few years exposure to air.

Travertine is another limestone-like carbonate-based rock that is often called either a marble or a limestone, but is acutally formed by a very different kind of sedimentary process. Unlike the true limestones and tufa, which are formed by (<i>CaCO<sub>3</sub></i>) deposits built up on the sea floor or on lake bottoms,respectively, travertine is (<i>CaCO<sub>3</sub></i>) that has been deposited directly on land, or sometimes in caves. Travertine may form from the mineral-rich water that flows to the surface from hot springs, or from surface water that has drained through limestone strata in regions known as karst formations. Deposition of the dissolved (<i>CaCO<sub>3</sub></i>) may be the result of several different processes, including evaporation or changes in temperature and/or acidity of the water.

The uneven surface of a growing travetine mass will often host deposits of moss and other matter that are eventually encapsulated, leaving the the stone riddled with holes and cavities. Stalactites and stalagmites are travertine deposited underground.


Marble has traditionally been regarded as the most perfect carving stone, for its beauty, its workability, and its capacity to take detail. It is quarried around the world, but many of the finest sculptural marbles are found in Italy. In the United States, marble is quarried in Vermont, Tennessee, Alabama, Colorado and elsewhere. The Italian marbles are unmatched for figurative carving, not only for their mechanical properties but because the translucence of is similar to that of flesh, giving a life-like appearance.

Stones in the marble family are in the middle range of hardness, but all are soft enough to work with steel hand tools. Mohs hardness numbers range from 3.5 to 5.0. All of the manual and power techniques work well with marble: chiseling, splitting off chunks and flakes with a point, bushing, rasping, scraping and drilling.

Block of Italian marble showing the marks of various tools.

Marbles tend to be somewhat translucent, polish well, and take fine details. Marble comes in many colors, and may be perfectly uniform in appearance, heavily veined, or anywhere in between. Marble sold for carving varies widely in appearance and carving properties, with some of the varieties being very hard and brittle. Commonly available marbles may be pure white, shot through with smoky dark veins, or even nearly black. Other varieties are brightly colored or heavily veined with colors.

The so-called Dolomitic marbles contain, in addition to calcium carbonate, large amounts of calcium magnesium carbonate. They are very similar in appearance and physical properties to the true marbles. All varieties are subject to inclusions of other mineral and flaws which may or may not be detectable from outside.


The limestones on the whole tend to be softer and coarser than marbles, although some limestones are harder than some marbles. Limestone is usually blander in appearance, often buff or gray in color. The stone is typically are in the Mohs 3.0 to 4.0 range of hardness. In general, limestone is usually relatively opaque, giving finished sculpture a deader look than marble, which tends to be slightly translucent. Because it is often so bland, sculpted limestone is more often seen on buildings than as free-standing artwork. Fossil sea-shells are sometimes found embedded in limestone. As with marble, there are "dolomitic" limestones composed of calcium magnesium carbonate but also as with marble, these are not easily distinguished from calcium carbonate limestones either by appearance or by physical properties.

Many varieties of limestone withstand weathering and acid rain better than marble, and some varieties harden slowly after quarrying, due to evaporation of entrained water. Such hardening proceeds slowly from the outside in.

Limestone tends to be easily worked with hand or power tools. While some limestone can take a polish, these stones tend not to keep a shine when exposed to weather.

Sculpture is a very minor use of limestone. It is heavily used in industry as a construction material and a the raw material for making cement, quick lime, and lime for plaster. It is also used as fertilizer and as a buffering agent for neutralizing acids.

Metropolitan Museum of Art, c. 1910, Detail of facade showing use of Indiana Limestone

In the United States, Indiana Limestone is the most commonly available type. It's color ranges from creamy tan to pale blue-gray, and the grain can be very even or varigated. It the most widely used architectural stone in the US, and is often carved for building ornamentation. Famous buildings constructed of Indiana Limestone include the Pentagon, the National Cathedral in Washington DC, and The Metropolitan Museum of art, The Empire State Building, and Grand Central Station in New York.

Granite and Related Igneous Stones

All of the stones in this family are igneous, being formed when large masses of molten rock (magma) orginating deep in the earth are forced to higher levels in the Earth's crust, where they cool and solidify into rock. The cooling depth can be anywhere from tens of thousands of feet down, to the earth's surface, and need not happen entirely at a single level. These rocks vary widely in chemical makeup, but but most are dense, hard, tough, extremely durable, and can be polished to a glassy finish. Stones that solidify deep in the earth are said to be "plutonic."

The appearance of these stones tends to be greatly affected by the depth at which they solidified from the molten state. Those that solidify deep in the earth cool slowly, giving them time to form large crystals of their component minerals. These stones (e.g., granite) tend to have a coarse visible structure, often differently colored grains fused into a single mass. The stones that solidify close to, or on, the Earth's surface (e.g. basalt) cool too quickly for large crystals of distinct minerals to form, and are thus finer grained and uniform in texture. The picture below is perhaps the most famous granite sculpture in the United States.

Mount Rushmore Presidential Memorial, granite, completed 1939, Gutzon and Lincoln Borglum

The great hardness of all of these stones, Mohs 6.5 to over 7.0, limits the techniques available for working it sculpturally. Most of the constituent minerals are harder than hardened steels, especially quartz, which is harder than any steel alloy. Steel tools strike sparks of burning metal when hit against these stones.

Despite their great hardness, these stones do tend to split well, with either heavy carbide chisels or pitching tools. However, even carbide chisels tend to crush these stones superficially, rather than penetrate. While chisels can be used to break off chunks, it is difficult to chisel away controlled amounts, as can be done with softer stones, so hand techniques that rely on pulverizing away the surface of the stone are often used. Punches, flat chisels, and bush hammers are used to crush the surface of the work-piece, gradually reducing the stone to powder.

One way to remove a lot of stone quickly, in a controlled way, is to use a diamond saw to slice the region of stone that is to be removed into thin "leaves" that can be subsequently broken off with chisel and hammer. Machine-powered diamond or tungsten carbide burrs, and silicon carbide (Carborundum) grinding tools, are used for surface shaping and smoothing.

The silica content of these stones varies, but they all produce a glassy dust that is damaging to the lungs upon chronic exposure. It was said in the old days, that there are plenty of old marble cutters, but very few old granite cutters. Therefore, stones in this category should not be worked at home or in schools, where adequate protection and cleanup are generally not practical.


The granites family are felsic (i.e., rich in feldspar26, quartz27, and mica28). These stones solidified deep in the earth, cooling very slowly, and typically have a very visible speckling from the large crystals of the various component minerals. It tends to be best for simple sculptural shapes, both because of the difficulty of carving fine details, and because detail is obscured by the speckles.

Six of the thousands of types of granite.

Basalt, Diabase, and Porphyry

Basalt, diabase porphyry are mafic, i.e., rich in magnesium and iron, and tend to have have similar working properties to granite, being dense, hard, and tough.

The magma deposits that form basalt cool either at shallow depth in the Earth, or are actually poured onto the surface, or onto the sea floor, by volcanos. In either case, they cool too quickly for large crystals to form, and thus have a fine-grained, blander appearance, somewhat like a dark limestone.

If the same magma that would form basalt if released close to the surface is injected more deeply in the earth's crust, the cooling is slower, and diabase, is the result. The slower cooling yields larger crystal sizes than are found in basalt, and a greater separation of the crystalized constituents. Basalt carving is most familiar from ancient Egyptian artifacts, but some contemporary carvers are using this stone too. A beautiful carving of a falcon by Tony Angel is shown below.

"Solitary Falcon, basalt, 9.5 x 8.5 x 11, 2013, by Tony Angel" (Photo courtesty of Foster/White Gallery.)

Porphyry is a diabase-like stone that is characterized by two-stage cooling. First, a long, slow cooling, at great depth, allows large crystals to form in a still-molten matrix. The preliminary phase is followed by a rapid cooling when a subsequent upward movement of the magma toward, or even onto, the Earth's surface. The rapid cooling freezes the molten matrix into tiny crystals, leaving the large crystals embedded in a homogeneous background. The mix of the two scales of crystallization gives porphyry a beautiful and decorative appearance.

Despite the hardness and toughness of these stones, incredibly, the ancient Egyptians were able to carved these stones by pulverizing away the surface by pounding it with balls of still-harder diorite. Diorite is a related, relatively rare, igneous rock that was slowly cooked above a subduction zone, where the cool crust is being slowly re-ingested into the Earth's molten mantle. More remarkably, the Egyptians also drilled and sawed these stones by using relatively soft copper and bronze tools to impart motion and pressure to abrasive sand, much as modern stone workers do with abrasive fiber wheels.

Two varieties of basalt are on the left, two varieties of diabase on the right. The stones are similar chemically, but diabase is grainier.


A more exotic igneous sculptural stone in this class is obsidian, which is a dark gray, green or black black volcanic glass, similar to quartz. Obsidian is composed primarily of silicon dioxide originating in magma, but cooled so quickly that it is amorphous, meaning that it has has no crystal structure at all, like glass or jello. Obsidian is harder than marble, but much softer than granite or basalt, having a Mohs hardness in the range of 5.0 to 5.5.

Obsidian's lack of crystalline structure causes it (much like ordinary glass) to fracture conchoidally, leaving a slippery smooth, undulating surface, and razor sharp edges--sharper than razors, actually. Flakes of obsidian have the sharpest edges known to science. The edge can be only molecules wide--as much as five times sharper than the finest steel surgical instruments. Once used by the American Indians for knives and arrowheads, obsidian flakes are now used in cardiac-surgery instruments because their extreme sharpness causes less cell damage at the microscopic level, resulting in faster healing. Obsidian being a form of glass, the carving dust is inherently dangerous, and proper protection is required. Because it fractures like glass, it can only be carved by grinding. It is generally available only in small pieces.

A typical chunk of obsidian (photo courtesy of Ji- Elle via French Wikipedia.)


Soapstone, also known as steatite (from the Greek word for fat), is an impure gray-green to black variety of the mineral talc. Pure talc has the distinction of being the the softest known mineral, and thus, has a Mohs hardness of 1.0. Baby powder is pure white talc. Because of its impurities, steatite is harder, having a Mohs hardness between 2.0 and 3.0 It is usually green, gray or white, but still has the slippery feel of baby powder. Soapstone is composed primarily of hydrated magnesium silicate, and is extremely easy to work.

A block of raw Italian soapstone for carving. The stone has been sanded to #220 and wetted.

Soapstone carvings can be treated with oil or left untreated. The material is not absorbent, and once oiled, tends not to stain easily. It is also resistant to heat, acid, and can be made into useful objects as well as sculpture. Bowls and other vessels, cook-tops, work surfaces for both kitchens and laboratories, and smoking pipes are all common uses. It holds heat and cold very well, which makes it useful in food preparation and serving.

It can be cut with a carpenters saw, hack saw, or coping saw, and can be carved with woodworker's tools, or even whittled with a pocket knife.

Because it is so easily worked with a rasp, soapstone is an ideal stone for young people, who can move up to edged tools and rotary tools, as their confidence develops.


Serpentine is a metamorphic rock, chemically related to soapstone, but more complex chemically and structurally, often occuring intermixed with other minerals. Strictly speaking, the rock should be called serpentinite, and the word serpentine reserved for the family of minerals of which sepentinite may be composed, but few other than geologists use the correct terminology.

Serpentine is harder than soapstone, but still easily worked, and is often sold for carving. Serpentine's Mohs hardness is usually around 3.0, which is too hard to be carved with a knife, but easily bushed, rasped, or chiseled. Old-fashioned square, so-called soapstone laundry sinks are usually made of serpentine, not soapstone. It is usually gray to green, and may be either partially translucent or opaque. Some varieties are fairly plain, and others have beautiful complex structure.

It is a common stone in many areas, but it's not a good idea to carve your local serpentine unless you are sure that it is free of asbestos, with which it often occurs. (Asbestos is one of the serpentine minerals.) The geology department of a local college may be able to help determine this.

A closeup of a green serpentine (Photo courtesy of kevinzim/Kevin Walsh via Wikipedia.)


More than one kind of stone goes by this name. The more common of the alabasters is a form of calcium sulphate, or gypsum--the raw material from which plaster of Paris and plaster-board (a.k.a. Sheetrock) are made. Chemically, it is CaSO4H2O, i.e., calcium sulphate with a water molecule chemically attached. This stone is translucent, almost transparent, and takes a high polish easily. It comes in a variety of colors, and many varieties are very beautiful, shot through with veins of color. It is among the softest stones, having a Mohs hardness 1.5 to 2.0--soft enough to whittle with a knife, and some varieties are soft enough to scratch with a fingernail.

Alabaster is mainly worked by rasping and grinding rather than with hammer and chisel due to the ease with which it bruises29

Various colors of alabaster can be purchased directly from quarries in the Western United States if you're willing to buy enough, or can be had in smaller quantities from sculpture supply houses.

Head of St. John the Baptist 15th Century England, Alabaster (calcite), Victoria and Albert Museum

Calcium sulphate alabaster is so pretty that using it sculpturally can be a challenge, as the intrinsic beauty of the stone tends to overpower the carving, and the translucence and veining can obscure detail. However, the softness of alabaster does make it less frustrating stone for beginners than marble or limestone. Alabaster is slightly water soluble and therefore does not weather well.

The other alabaster is a kind of calcite, which is a form of calcium carbonate, and is thus related to limestone and marble. Calcite alabaster is harder, app. Mohs 3.0. You can also tell which one you are dealing with by testing with hydrochloric acid. Calcite alabaster will foam, while gypsum alabaster is more or less unaffected.

Both kinds are mined in the United States. Gypsum alabaster is the variety more often sold for sculpture.


Anhydrite is chemically similar to alabaster, being pure calcium sulphate, CaSO4, without the attached water molecule present in alabaster. With a Mohs hardness of 3.0 tp 3.5, it is harder than alabaster, but softer than most varieties of limestone or marble. Colors range from white to dark brown, and it can be variegated like alabaster, but is usually less showy.


Sandstone is composed of sedimentary sand cemented together in a matrix of other minerals under the pressure of overlying deposits. Many forms of sandstone are carvable with the right tools. The sandstones vary widely in overall hardness, color and durability out of doors, but all are extremely abrasive due to the hard quartz sand.

Sandstones also vary widely in their ability to hold detail, due to their granular nature. The "brownstone" neighborhoods of New York and other cities in the Eastern US, dating from the latter half of the Nineteenth Century, are (or more accurately, were) mostly faced with Triassic-era sandstone from New Jersey and Connecticut, often extensively carved. These sandstones are composed of silica sand cemented with iron oxide, which gives it its distinctive brick-red color. The lack of resistance of this stone to weathering is evident from the ubiquity of the artfully applied stucco with which much of it has been re-faced in the last fifty years. How much varies from neighborhood to neighborhood, but in most areas, almost all the original sandstone has severely eroded and has been covered or removed.

Carved sandstone ornamentation on a Manhattan brownstone, c.1885

There seem to be two main modes of erosion: sometimes the stone crumbles away from the surface, rounding off the details. More destructively, water gets into the stone, and cause it to swell like a book left out in the rain, exfoliating large flakes and chunks. Sandstone can withstand the elements for many years, seemingly impervious, then suddenly disintegrate in just a few years. The differences in erosion patterns may be caused by differences in the stone itself, the orientation of the carved stone with respect to it's bedding direction, subtleties of placement that result in better or worse drainage, more or less exposure to the sun, resulting differences in the freezing and thawing patterns, etc.

Sandstone is widely available, and in many areas is quarried locally. Beware of sandstone you find or quarry yourself, because if it's exposed enough for you to find it, it may be weathered to an upredictable depth. The hardness and durability of varies widely. Connecticut/New York/New Jersey brownstone weathers fairly quickly, but the sandstone sold in New York as "blue stone" is very durable; New York blue stone paving stones a century old seem almost unaffected by weathering, and many have withstood repeated resetting because of heaving by tree roots. The grainy nature of sandstone makes some varieties unsuitable for finely detailed carvings, but some varieties do hold detail, and can even be polished, such as the gray-green graywack sandstone carved by the Egyptians. The image below is carved from a dense, fine-grained Egyptian sandstone, which unlike some sandstones, polishes and takes detail well. Note that hundreds of varieties of sandstone are called graywack, and many do not have the uniformity and other characteristics of the Egyptian stone.

Apollo with Lyre--Greywack from Wadi Hammamat (Egypt), 1st C. CE copy of the cult statue from the temple of Apollo in Circo (179 BC)

Sand is silica, which means that you need to observe the lung-safety precautions covered elsewhere in this book, and it probably should not be carved in a home or schools studio. Whether hard or soft, the quartz content (Mohs 7) makes sandstone extremely abrasive, so tungsten carbide is usually used, as it will wear down steel tools very quickly.


Slate, familiar in the form of roofing shingles and blackboards, is metamorphic, originating as shale, a sedimentary rock which in turn originates as clay. It is found in several of the Northeastern states, and while usually encountered in sheets, can also be obtained in carvable blocks.

"Ravens Wall, slate, 48x18, 2007, by Tony Angel" (Photo courtesty of Foster/White Gallery.)

Slate is unsuitable for outdoor pieces because it weathers quickly, splitting into thin sheets. (The slate used for roofing always has the bedding direction parallel to the roof, so it does not absorb water. This would not be true of a block.) It is not carved with chisels due to its fragility, but like alabaster, can be sawn and ground. Slate typically has a Mohs hardness of about 3.5, similar to a soft limestone. The tendency to break into sheets is a good example of anisotropy, a property that is not captured by the Mohs scale.

African Wonder Stone (Pyrophyllite)

African wonder stone, a form of the mineral pyrophyllite, is somewhat similar in it's working properties to soapstone, and is available from sculpture supply houses. Pyrophyllite (so named because it breaks into leaves on exposure to flame) do not have the silky touch of soapstone and talc, and is deader and more clay-like in appearance. Wonder Stone is a homogeneous, not very interesting dark gray, but it can be sealed to a silky, almost black finish. It carves very easily and is a good stone for beginners, but it generates a lot of dark gray dust.

African wonder stone figure carved by Larry Ahvakana (Photo courtesty of Stonington Gallery.)

Pyrophyllite occurs in many other colors as well and is found in a number of locations in the United States, but is not often encountered in raw form, being mined primarily for the ceramics industry and as a refractory material. It is harder than soapstone, and having a Mohs number of about 3.0, is soft enough to be sawn by hand or worked with a steel knife.