Onyx and Variations
Onyx is the name given to a gem variety of Agate (Chalcedony). Agate is the name given to a family of chemically related stones. There are over thirty distinct varieties of agate making it one of the largest families of stones. All agates are composed of SiO2 They form according to a trigonal crystal structure, and have a hardness of 7 on the Mohs scale.