mineral

Black Onyx (Tumbled)

Chalcedony (Silicon Dioxide, SiO2)

Hardness: 6.5-7 on Mohs scale. Color: Solid black (often dyed for uniformity). Luster: Vitreous to waxy. Crystal structure: Cryptocrystalline. Cleavage: None (Conchoidal fracture). Specific Gravity: 2.65-2.67.

Hardness: 6
Color: Solid black (often dyed for uniformity)
Luster: Vitreous to waxy

Identified July 20, 2026More mineral →

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Physical properties

Formation & geological history

Forms in the cavities of gas bubbles in volcanic lava. Over time, silica-rich fluids deposit layers of microcrystalline quartz. Most black onyx in the market is natural chalcedony that has been treated with a sugar-acid carbonizing process to achieve a deep black color.

Uses & applications

Extensively used in jewelry (rings, beads, cameos), decorative carvings, and as a healing stone in metaphysical practices.

Geological facts

Pliny the Elder described techniques for darkening chalcedony to create onyx in ancient Roman times. Historically, onyx was used for seals and intaglios because it does not stick to wax.

Field identification & locations

Identified by its waxy luster, smoothness, and extreme hardness (it will easily scratch glass). Look for it in riverbeds or volcanic regions, though solid black specimens are rarely found in nature without some banding.

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