stone types

Types of Stone for Carving

Stone:	Color:	Hardness:	Respirator Required (X)
Igneous
Granite	Gray, black, browns, reds, greens, gray, blue-gray	Very hard	X
Sedimentary
Limestone	Gray, buff, variegated gray/buff	Soft
Sandstone	Buff, reddish brown, gray	Medium to Hard	X
Metamorphic
Marble	White, grays, greens, reds, black, variegated	Medium
Soapstone	Grays, black	Very Soft	X
Alabaster	White, grays, beige, orange, yellows, reds, white translucent, variegated	Very Soft

Note: The Sculpture Studio does not sell stone or tools.

Man has been creating art from stone ever since he found he could shape it by striking a softer stone with a harder one. Over the millennia, a few types of stone have become popular with sculptors. Here are the most common ones used for carving from the three different types of rock: igneous, sedimentary, and metamorphic. Igneous: The characteristics of igneous rocks result from the way in which they were formed. Deep in the earth, under the intense heat of volcanic action, magma was forced up through the older solid rock. It then cooled, forming granite, basalt, and diorite. Granite is made of quartz crystals and feldspar which contain silica. Prolonged breathing of silica can cause lung damage. Wear a good respirator when around granite dust. Granite is very hard stone and difficult to carve. In fact 'carving' might not be the right word, since you are not pushing the chisel through the material like you would with the limestone or marble, but are pulverizing the stone, trying to break off chips of crystals (which can be as sharp as glass). Your carving tools must be carbide-tipped to stand up to the wear and tear of working granite. Fine detailed carving is better left to other types of stone. Diamond saws and cup grinders cut through granite quite easily and can speed up the carving and finishing process. Granite comes in a wide variety of colors, and the crystal size can vary from large and coarse to very fine and dense. Granite takes a high polish and holds up very well outdoors. Sedimentary: As rocks began to erode from wind, rain, and sun, the particles were washed into low lying areas where the sediment accumulated. In the sea, small plant and animal forms died and drifted to the bottom, adding to the sediment. Over thousands of years, the pressure of these layers cemented the sediment onto limestone or sandstone. Limestone formed on the sea floor from sediment and the bodies of primitive sea creatures. As you are working, you can find small fossilized creatures like crynoids and braciopods in the stone. Limestone is composed primarily of the mineral calcite, or calcium carbonate. Limestone is easy to carve and will hold small detail work, but is also strong enough to support undercutting. While it does not have the innate beauty of the more colorful stones, a wide variety of textures can be created that can really bring the stone to life. There is a definite 'grain' or bed (formed as the layers of sediment stacked upon each other on the sea floor) in limestone. The stone breaks fairly predictably along the bed lines, but less cooperatively going across the bed. (Think of opening a phone book with the pages, compared to trying to tear it across the pages.) Limestone can be polished, but the polish will not last long outdoors. Limestone seems to survive acid rain better than marble, so is a good choice for outdoor sculpture. Sandstone is formed from sedimentary sand held together by silica or calcium carbonate. Sandstone also has a distinct bed direction, and large chips can be broken off when going with the bed. Sandstone tends to wear out your tools quickly. Sandstone contains silica, so a respirator must be worn. Metamorphic: Metamorphic rocks are formed when a sedimentary layer is exposed to heat and pressure and undergoes a chemical change which forms a new crystalline material. After metamorphism, limestone becomes marble. Marble has been the most preferred stone for carving since the time of the ancient Greeks. Marble is moderately hard to work. It will hold very fine detail. Marbles from the United States comes in over 250 colors. When brought to a high polish, its crystalline structure sparkles. It does not hold up well outdoors. Acid rain begins to deteriorate the surface within a few years. Alabaster is a very soft stone for carving and tends to flake and split along hidden cracks in the stone. It will take a high polish, which brings out its incredible colors and patterns. In fact the stone is so beautiful that the viewer may overlook your sculptural forms and only admire the stone. The dust from alabaster may cause an allergic reaction in some people, so wear a respirator. Soapstone, or steatite, is soft enough to carve with a knife. It is composed of talc and has a slippery, soapy feel. It will take a polish and hold fine texture detail. It is a good choice for your first stone carving. Prolonged exposure to talc dust can cause respiratory problems, so wear a respirator.