One slab order is at least five square inches of black chalcedony (a black onyx) from Africa. This semi-precious gemstone is a popular and classic color for jewelry and will take a nice polish. Another use for black chalcedony onyx is as a base to apply opal to, and this will brighten the colors of the opal. Clear quartz is then applied to the top of the opal cabochon to protect the soft opal.