Onyx

Onyx is a variety of chalcedony quartz characterized by its parallel bands of color. While many people think of onyx as only solid black, it naturally occurs with black and white or brown and white bands. It is distinguished from agate by the fact that agate has curved (concentrically) bands, while onyx features straight, parallel bands. It has been used for cameos and carvings since ancient times.

Formation & Geology

Onyx forms in the gas cavities of volcanic rocks (lava). Silica-rich fluids deposit layers of quartz over long periods. The different colors of the bands are caused by trace impurities as the layers are deposited. Most solid black onyx sold today is actually gray chalcedony that has been dyed, a treatment that has been practiced for over 2,000 years since the Roman era because natural solid black onyx is extremely rare.

Physical Characteristics

Onyx has a hardness of 6.5 to 7 and no cleavage. It has a vitreous to silky luster and can range from translucent to opaque. Its parallel banding makes it the ideal material for 'cameo' carving—artists can carve through a white layer to reveal a black background, creating beautiful high-relief images. It is tough and resistant to wear, making it suitable for rings and everyday jewelry.

Historical Significance

Onyx was highly prized in ancient Egypt and Rome. It is mentioned in the Bible as one of the stones in the High Priest's breastplate. In Roman times, it was used specifically for seals because wax would not stick to it. The name 'Onyx' comes from the Greek word for 'fingernail' or 'claw', likely because of the white layers that resemble the lunula of a human nail. It is a traditional birthstone for July or December in various cultures.

Black Onyx vs. Dyed Agate

True natural black onyx is exceptionally rare. Since Roman times, the vast majority of "black onyx" on the market has actually been gray chalcedony or agate that is soaked in a sugar solution and then treated with sulfuric acid to carbonize the sugar, turning the stone black. This treatment is permanent and has been accepted for over 2,000 years, but it means your black onyx ring is likely "cooked" agate.