Onyx

Hardness: 7 on Mohs scale; Color: Black, brown, yellow, green, red, pink, white, often banded or mottled; Luster: Vitreous to waxy; Crystal Structure: Trigonal (microcrystalline quartz, forms in masses not well-defined crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.58-2.64

Onyx is a variety of chalcedony, which is a microcrystalline form of quartz. It forms from silica-rich waters depositing successive layers of mineral. It is often found in the gas cavities of lava flows, where silica solutions precipitate in layers, creating the characteristic banding. Its geological age varies greatly depending on the specific deposit, occurring throughout Earth's history where volcanic activity and silica precipitation occurred.

Primarily used in jewelry (cabochons, beads, carvings), ornamental objects, and decorative items. Black onyx is a popular choice for mourning jewelry and men's rings. Due to its banding, it is also used for cameos and intaglios.

Onyx has been used since ancient times for carving and jewelry. The name "onyx" often refers to sardonyx, which has bands of sard (reddish-brown chalcedony) and white. Banded onyx is typically dyed to enhance its color or create consistent black varieties.

Field identification often relies on its characteristic parallel banding, conchoidal fracture, and vitreous to waxy luster. It can be distinguished from agate by its more straight and parallel banding. Common locations include deposits in Mexico, Brazil, Argentina, India, and the USA (California, Arizona). For collectors, look for well-defined banding and vibrant colors, though natural black onyx is rare; most black onyx on the market is dyed.