Onyx is a parallel banded variety of chalcedony, a silicate mineral. It is an ancient gemstone that has been used in jewelry and as an ornamental gemstone for carvings since antiquity. Onyx has traditionally been one of the most important engraving and cameo gemstones, where its most common form was a black base with a white upper layer. Although still used as a minor gemstone today, the significance of onyx has waned since ancient times.

Onyx is a type of chalcedony, which is itself a form of microcrystalline quartz. Onyxes have straight, nearly parallel bands or layers of color, which allow skilled gem carvers to cut away material to create cameos and intaglios with extraordinary depth and contrast.

Onyx is often used in jewelry such as rings, necklaces, and bracelets. It is also used for decorative purposes such as vases and bowls. Black onyx is the most common variety of onyx used in jewelry.

In conclusion, onyx is a beautiful gemstone with a rich history. Its unique parallel banded structure makes it an ideal choice for cameos and intaglios. Its use in jewelry and decorative objects continues to this day.