Onyx

Onyx is a banded variety of chalcedony, the microcrystalline (cryptocrystalline) form of quartz. True onyx has straight, parallel bands, classically alternating black and white, which distinguishes it from the curved banding of agate.

In modern jewelry the name 'onyx' most often refers to solid jet-black chalcedony, much of which is dyed to deepen and even out its color, a treatment that has been practiced since antiquity. When the bands are brown or reddish and white, the stone is called sardonyx.

Valued since ancient Egypt, Greece, and Rome, onyx has long been carved into cameos, intaglios, seals, beads, and ornamental objects, and remains a popular, affordable black gemstone today.

Onyx forms like other chalcedony, by the slow deposition of silica from groundwater into cavities, cracks, and gas pockets in rocks, especially volcanic rocks. Successive layers of differently colored silica build up the characteristic parallel banding.

The flat, parallel layering (versus the concentric, curved banding of agate) reflects deposition on the floors of cavities in roughly horizontal sheets.

Natural onyx and sardonyx are found in Brazil, India, Uruguay, Madagascar, and the United States, among other places. Note that 'onyx marble' (a banded calcite/aragonite used for carvings and countertops) is a completely different, softer carbonate rock, not true quartz onyx.

Look for a hard, fine-grained stone with a waxy to vitreous luster, either solid black or showing straight parallel black-and-white (or brown-and-white) bands. Hardness is 6.5-7, so it scratches glass, and there is no cleavage; it breaks with a conchoidal fracture. The streak is white.

The straight, flat banding helps separate onyx from agate, which has curved or concentric bands.

Look-alikes include dyed black agate or glass (glass may show bubbles and is warmer to the touch), obsidian (glassy, lower hardness around 5-5.5, often with conchoidal sheen but amorphous), and jet (much softer, lightweight, warm, can be carved easily). Onyx's hardness, waxy chalcedony texture, and flat bands are key.

Onyx is a popular and affordable gemstone, cut into beads, cabochons, and inlays and especially favored for men's jewelry, signet rings, and mourning jewelry because of its deep black color. Sardonyx and banded onyx have been carved into cameos and intaglios for thousands of years, using contrasting layers for raised designs.

It is also fashioned into ornamental objects, boxes, and decorative inlay. Most commercial black onyx is dyed to achieve uniform color, a long-accepted and stable treatment.

Metaphysically onyx is marketed as a protective, grounding stone for strength and focus; these are spiritual associations rather than proven properties. Beware that 'onyx' sold for countertops and carvings is usually the softer carbonate onyx marble.