hardstone

"Hard stone" refers to an opaque, usually semiprecious stone that can be shaped or carved, often used for jewelry or mosaics. The Italian term pietra dura translates to "hard stone" and describes stones with a Mohs hardness between 6 and 10, used in intricate mosaic art, particularly in Florence. These stones are valued for their hardness and durability. Examples include quartzes, chalcedonies, agates, jaspers, granites, porphyries, and petrified woods. Hardness in minerals is their resistance to scratching, a key factor in identification. Claystone is a hardened clay, and some geologists define it as a sedimentary rock composed mainly of clay-sized particles that are not easily split into layers.1234