Onyx (beads)

Hardness: 6.5-7 on Mohs scale. Color: Often black, but also white, brown, red, and banded varieties (then called sardonyx). Luster: Waxy to vitreous (glassy). Crystal structure: Trigonal, microcrystalline. Cleavage: None, exhibits conchoidal fracture. Specific Gravity: 2.65 - 2.66.

Onyx is a cryptocrystalline variety of quartz, similar to agate, chalcedony, and jasper. It forms in the gas cavities of lava, where silica-rich groundwater deposits microscopic quartz crystals in concentric layers. The black color in black onyx is often achieved by dyeing natural gray onyx or chalcedony. Geological Age: Varies widely, depending on the volcanic activity and hydrothermal conditions, but generally found in formations from various geological ages.

Primarily used in jewelry (beads, cabochons, carvings), ornamental objects, and often carved into signet rings and cameos due to its fine texture and ability to take a high polish. While less common in construction, it can be used for decorative tiles or inlays.

The name 'onyx' comes from the Greek word 'onux', meaning 'fingernail' or 'claw', possibly due to its translucent, flesh-toned variety or its alternating bands resembling an unpolished fingernail. Ancient Romans and Greeks used onyx extensively for carvings. Sardonyx, a variety of onyx with parallel bands of sard (reddish-brown chalcedony) and white, was highly valued by the Egyptians and Romans.

In the field, natural onyx is found in volcanic regions. Identifying it among other chalcedony varieties can be challenging without examining banding. Black onyx is characterized by its uniform deep black color and waxy luster when polished. Its hardness means it can scratch glass but will be scratched by harder minerals like corundum. When identifying beads, look for consistent color, absence of flaws, and a good polish. Natural black onyx is rare; most commercially available black onyx is dyed. To differentiate from obsidian (natural volcanic glass), examine for internal translucence (onyx can be slightly translucent on thin edges) and its conchoidal fracture which, while present in both, can appear slightly different due to onyx's microcrystalline structure.