Talc is a metamorphic rock and the softest mineral on Earth. It is so soft that a fingernail will scratch its surface, and geologists can recognize it by its greasy feel. Talc is a hydrous silicate of magnesium, Mg3Si4O10(OH)2, and usually contains small quantities of nickel, iron and aluminum as impurities. It occurs commonly in thin layers known as foliafolia, but is also found in coarsely granular, finely granular crystalline masses. Talc appears transparent to opaque, is rarely colorless, and can be found in hues ranging from white to purple. One form of this mineral is soapstone, or steatite, which is a larger granular form of talc that is also rich in magnesium.