Mohs scale of mineral hardness
The definition of "hardstone" is not very rigid, but excludes "soft" stones such as soapstone (steatite) and minerals such as alabaster, both widely used for carving. Hard organic minerals such as amber and jet are included, as well as the mineraloid obsidian. Geologically speaking, most of the gemstones carved in the West are varieties of quartz, including chalcedony, agate, amethyst, sard, onyx, carnelion, heliotrope, jasper and quartz in its uncoloured form, known as rock crystal. On the Mohs scale of mineral hardness, quartz rates no higher than a 7, less for types with impurities. Stones typically used for buildings and large sculpture are not often used for small objects such as vessels, although this does occur.