TYPES OF STONE
There are three general types of stone: sedimentary, igneous, and metamorphic. The type of geologic forces that produce the stone determines the variety:
SEDIMENTARY STONES are made of particles that were laid down over a period of time in a series of layers. Stones of this type which are commonly used for carving are sandstones and various limestones, including travertines, onyx marbles, and some limestones classified commercially as marble. Sedimentary stones will often have visible fossils within them.
IGNEOUS STONES are made of material that was originally molten, and the stone was formed as the material cooled at or near the Earth's crust. Igneous stones are generally harder and more difficult to work than sedimentary stones, although that same hardness and difficulty also means a more durable finish for the sculpture. Some of the more commonly carved igneous stones are granite, basalt, and to a lesser extent, obsidian.
METAMORPHIC STONES were originally either sedimentary stone or igneous stone that was changed over long periods of time by the heat and pressures of geologic forces. The types of metamorphic rock that are commonly used for carving are marble, soapstone (including steatite and talc), serpentine, and slate.
Besides the types mentioned above, other rocks and minerals which are sometimes carved for sculpture are: amber, alabaster, jade, lapis lazuli, malachite, and various types of quartz; including agate, bloodstone, chalcedony, jasper, and true onyx.
THE ORIGINAL MOHS' SCALE OF HARDNESS
talc gypsum calcite fluorite apatite orthoclase quartz topaz corundum diamond
In 1812 Friedrich Mohs, an Austrian mineralogist, developed a scale consisting of ten relatively common minerals showing an order of increasing relative hardness. Each listed mineral can scratch those with a similar or lower number, but cannot scratch higher numbered minerals. To put it in a better perspective: a human thumbnail is about 2-1/2 on the scale; a copper penny is 3; a steel knife blade is about 5-1/2; and a hardened steel file is 7. The numbers on the scale do not indicate equal measures of hardness - if the scale was to be shown in proportional relationships, #1 talc would be the width of a hair, while #10 diamond would be as wide as this page.
Most of the types of carving stone discussed here will refer to this scale.
SEDIMENTARY STONES USED FOR CARVING
LIMESTONE is made out of calcium carbonate material deposited at the bottom of a lake or a sea. The calcium carbonate deposited on the sea floor consists primarily of the skeletal remains of microscopic marine life, although larger fossils are not uncommon. When currents constantly stir the sea floor, the calcium carbonate particles often clump together, forming little balls before further deposits bury them. This condition forms oolitic limestone, which is usually quite porous. Indiana limestone is an oolitic type, and weighs between 140 to 150 lbs. per cubic foot. On the Mohs' scale of hardness, it is 2-1/2. The Empire State Building, the National Cathedral, and the Georgia State Capitol Building are a few examples of Indiana limestone.
Some types of limestone are dense enough that they can be polished. Though sold commercially as marbles, stones such as Belgian Black, Radio or Champlain Black, and the red Spanish Rojo Alicante are all geological limestones because they have not been re-crystallized into true marbles.
TRAVERTINE is calcium carbonate limestone formed in layers by precipitation at the bottom of shallow hot spring pools. It has numerous distinctive pores and cavities, but the surrounding stone is capable of taking a polish so it is often sold commercially as marble.
ONYX MARBLE is a calcium carbonate limestone formed in layers by slow precipitation at the bottom of cold springs. Usually translucent because of the extremely fine crystalline structure, it most often is layered in pale shades of brown, yellow, and green. It can be polished like a true marble. Note that it is also not true onyx, which is much harder and is a type of crystalline silica closely related to agate.
SANDSTONE is a sedimentary stone consisting of grains of sand cemented together in a matrix of another mineral. It is usually quite porous and incapable of taking a polish. Sandstones are often easy to carve, although the hard particles of sand will dull the cutting edges of the tools very quickly. Sandstones are traditionally used for making sharpening stones and grinding wheels, although in recent decades synthetic stones have replaced the use of natural sandstone.
IGNEOUS STONES USED FOR CARVING
GRANITE is probably one of the most well known igneous rocks. It is a very hard, dense and coarse-grained crystalline stone composed of quartz, feldspar, and often mica. It has a very low porosity and is not subject to attack by acids, so it is one of the most durable stones for outdoor use. The granite mass of Georgia's Stone Mountain was originally deep underground, and was only exposed when the surrounding sedimentary and metamorphic rocks eroded away. Granite is about 61/2 to 7 on the Mohs' hardness scale (equivalent to the hardness of a steel file), and most varieties weigh between 160 to 170 lbs. per cubic foot.
BASALT, DIABASE, GABBRO, DIORITE, and ANORTHOSITE are all dense, hard, fine-grained crystalline rocks, usually ranging from dark brown, dark green to black in color. Commercially these stones are classed and sold as black granite.
MARBLE AND OTHER METAMORPHIC STONES USED FOR CARVING
MARBLE deposits are formed by a metamorphic re-crystallization of existing deposits of calcium carbonate limestone. During geologic periods of mountain building, the heat and intense pressure of the process cause the limestone to crystallize. True metamorphic marble usually exhibits a crystalline sparkle when it is fractured, and is translucent when cut into thin panels.
The geological definition of marble is crystallized limestone, but the commercial definition of marble encompasses all stones capable of taking a polish within the hardness range of 3 to 4 on the Mohs' scale. Though not as specific, the commercial definition is perhaps closer to the original meaning of the word. The word marble derives from the Greek word meaning bright or shining. Most of the white marble in Georgia and elsewhere is pure calcium carbonate. White marble is usually the mineral calcite, which is close to pure calcium carbonate. Dolomitic marble has magnesium carbonate either in addition to, or instead of, the calcite. It also usually is either partially or completely white. The various colors and veins in marble are due to chemical impurities, and quite often the colorful and highly veined varieties are harder and more difficult to carve than the white varieties. The presence of large amounts of organic matter in the original sediments causes the marble to be black, while iron-rich minerals produce red and pink marbles. Green marbles are not usually true calcite or dolomitic marbles, but are instead serpentine - which is a hydrated magnesium silicate, often veined with calcite, dolomite, or magnesite.
Like limestone, marble is particularly subject to corrosion from acids - even small amounts of acid in rainwater can quickly damage the surfaces of any exposed stone.
Although many of the fifty United States quarry marble, large deposits of white marble are in only three states: Vermont, Colorado, and Georgia. Although not a major producer, Alabama has a fine-grained white marble that in many ways is very similar to the white marble of Carrara, in northwestern Italy, which is what Michelangelo used.
Italian marble has been quarried for thousands of years, and today Italy is one of the world's major producers, yearly extracting about one third of all marble quarried worldwide. In addition to the 150 or so varieties of marble quarried in Italy, raw marble from around the world is imported into Italy for production work, and a major portion of the world's marble production is processed in Italy even though it is not quarried there.
GEORGIA MARBLE usually refers to the most common type of calcite marble quarried near Tate, in Pickens County. The marble is formed of large, dense crystals which give the stone a low degree of porosity - and this makes Georgia marble one of the better types of true marble for outdoor sculpture. Georgia marble is about 31/2 on the Mohs' scale of hardness, and weighs about 170-180 lbs. per cubic foot.
Georgia marble was formed from sediments laid down about half-a-billion years ago, during the Cambrian period. About 1840, Henry Fitzsimmons, who began quarrying it in 1842, discovered a surface outcropping of Georgia marble. Though Fitzsimmons died only three years later, other quarries were opened and in 1884 the Georgia Marble Company was formed.
Today Georgia is one of the nation's largest marble producing states, and has the resources to be so for the next several centuries. Commercially, less than half of Georgia marble is quarried as dimensional stone. Dimensional stone is quarried in large blocks that are cut into slabs or smaller blocks for architectural and sculptural use. Other than dimensional blocks, most marble is quarried by blasting into boulders which are crushed and used in other products - from gravel and garden lime to toothpaste and cosmetics, or from paints and plastic plumbing pipe to chewing gum.
The types of marble quarried near Tate are white, gray, or pink (or combinations of these colors). The black streaks that are often present are biotite mica, and the less common green streaks are another form of mica called fuchsite. Mica is not quite as hard as calcite, and its structure is of layered sheets. During carving the dark mica veins usually work faster, but then finish poorly compared to the surrounding calcite. North of Tate, near the town of Whitestone, the Georgia Marble Company quarries a dolomitic marble that is rarely used for sculpture, primarily because it is not quarried as a dimensional stone.
There are also other types of marble in Georgia, but quarriers do not find it as profitable to market dimensional blocks of marble as much as other forms of calcium carbonate.
At the current rate of quarrying, the now known deposits of Georgia marble will last for several centuries - one reason the U.S. government decided to specify that grave markers in National cemeteries be made from Georgia white marble.
SERPENTINE is a hydrous magnesium silicate, often green to greenish black in color and sometimes veined with lighter-colored calcite. Because it is usually within the hardness range of true marble and it can be polished; it is often called marble. Serpentine will not be affected by acid, except for occasional veins of calcite.
SLATE is a microcrystalline metamorphic rock that is derived from shale. It is composed chiefly of micas, chlorite, and quartz. Because of the orientation of the crystalline structure, there is a well-defined cleavage that allows slate to be split easily in the bedding plane, but not across it. Slate is not affected by acid.
OTHER STONELIKE MINERALS USED FOR CARVING
ALABASTER uses the same basic tools and techniques as for working marble, but alabaster is much softer and easy to carve, making it a good choice for novice carvers. The largest commercial deposits of carving alabaster in the U.S. are in the southwestern states of Colorado, Utah, New Mexico and Arizona. Alabaster is primarily made up of the mineral gypsum and is formed during geologic evaporative sequences.
In addition to its use as carving stone, gypsum alabaster is important to sculptors for another reason. When alabaster is calcined, or burnt to ashes, and then crushed into powder, it becomes one of the sculptor's most commonly used materials; plaster.
SOAPSTONE encompasses a wide variety of rocks, which contain the mineral talc (a hydrous magnesium silicate). Pure talc is white, and has a hardness of 1 on the Mohs' scale, so it can be scratched with your fingernail. Soapstones that have a high talc content are steatite. The color of soapstone varies widely, though most of the common varieties are green to blue black - usually the darker the color, the harder the stone. Soapstone is heat-resistant, and a variety of heating and cooking stoves are made from it. Some soapstones contain the mineral asbestos, so make sure you know what you are working with and take proper safety precautions. The tools and techniques used to carve soapstone are similar to what would be used for alabaster. The soapstone from Virginia, Alberene stone, is a homogenous medium-grained dark, bluish gray and is very good for carving - it is asbestos-free. Georgia has deposits of soapstone near Chatsworth, and until recently it was quarried there primarily for making crayons to mark on steel. The Georgia soapstone varied from a soft, pale emerald green stone to a hard, dark greenish black stone with a pronounced cleavage plane.