Soapstone (also known as steatite or soaprock) is a talc-schist, which is a type of metamorphic rock. It is largely composed of the mineral talc and is thus rich in magnesium. It is produced by dynamothermal metamorphism and metasomatism, which occurs in the areas where tectonic plates are subducted, changing rocks by heat and pressure, with influx of fluids, but without melting. Soapstone is composed dominantly of talc, with varying amounts of chlorite and amphiboles and FeCr-oxides. It may be schistose or massive. Soapstone is formed by the metamorphism of ultramafic protoliths and the metasomatism of siliceous dolostones.
Pyrophyllite, a mineral very similar to talc, is sometimes called soapstone in the generic sense since its physical characteristics and industrial uses are similar, and because it is also commonly used as a carving material. However this mineral typically does not have such a soapy feel as that from which soapstone derives its name.
Steatite is relatively soft because of the high talc content from which it was originally composed has a definitional value of 1 on the Mohs hardness scale. Softer grades may feel soapy when touched, hence the name. There is no fixed hardness for steatite because the amount of talc it contains varies widely, from as little as 30% for architectural grades such as those used on countertops, to as much as 80% for carving grades. Common, non-architectural grades of soapstone can just barely be scratched with a fingernail and are thus considered to have a hardness of 2.5 on the Mohs scale. If a candidate rock cannot be scratched with a knife blade (hardness of 5.5), it is not soapstone.