Onyx is often confused with marble, yet it is a significantly different rock type. Onyx is a sedimentary rock, formed as stalactites and stalagmites in cave interiors. This formation method results in the cryptocrystalline construction of the rock fabric, and it is the size and uniformity of these crystals that contribute to the classic translucent property of most onyx varieties. While vulnerable to chemical and abrasive attack, the decorative appeal of onyx is perhaps unsurpassed by any other material.