Enlightenment of Carving Stone (by a sculptor, not a geologist)
There are thousands of possible stones to carve. Each has its own characteristics. They can be classified by hardness, workability, “bedding” plane, and chemical composition. There is a hardness chart, the MOH scale, from 1 (soapstone) to 10 (diamonds). The latter is more than a thousand times harder than the former meaning it is a geometric (non-linear) scale.
Soapstone (1) is talc and can be scratched by a fingernail. It is easy to work and is weather durable. However, it obviously scratches easily and sometimes contains asbestos.
Alabaster (1.5) is calcium sulfate or gypsum AND 3
parts water. It carves well and finishes beautifully but
must be stored indoors (or you’ll get drywall dust). It comes in a wide variety of colors and translucencies and mostly as smaller boulders.
Steatite (2) is talc with some magnesium mixed in, which gives it a little more hardness. Like its cousin, it carves nicely with a rare soft spot and can be kept outside. Common colors are green, black, and gray.
Limestone (2-3.2) is a sedimentary stone of crushed shells. It carves nicely, the softer ones may not take an edge or achieve a high polish. It is opaque and is generally tan, light yellow, or gray.
Travertine (3-3.4) is calcium that formed from hot bubbling water and therefore has holes. Still, it can carve as well as marble and take a polish. Reds, yellows, creams, and whites are most common.
Marble (3-4.2) is compressed limestone or calcium carbonate. It might contain slight amounts of other minerals that give it veins or color (eg. copper for blue or green, iron for red, magnesium for orange or yellow, etc.) It can almost always take a fine edge and a high polish. Crystal size and layering (called bedding plane) vary greatly.
Onyx and Calcite (4) are, again, calcium carbonates formed by flowing water or dripping water (stalagmites/stalactites), respectively. They are usually translucent and finish beautifully. These are mostly cutting and grinding stones as they are too brittle for chisels
For the not faint of heart are the igneous stones of basalt and granite (6) which produce fine work and may contain minerals requiring good masks.