sedimentary
Banded Calcite (often sold as 'Mexican Onyx' or 'Green Onyx')
Calcite (Calcium Carbonate, CaCO3)
Hardness: 3 on Mohs scale (can be scratched by a copper coin); Color: Banded layers of cream, light green, and brown; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral; Specific gravity: 2.71.
- Hardness
- 3 on Mohs scale (can be scratched by a copper coin)
- Color
- Banded layers of cream, light green, and brown
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale (can be scratched by a copper coin); Color: Banded layers of cream, light green, and brown; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral; Specific gravity: 2.71.
Formation & geological history
Formed through the chemical precipitation of calcium carbonate from cold spring water or groundwater in caves (travertine) and vein deposits. Geological age varies by site but many commercial deposits are from the Cenozoic era.
Uses & applications
Used extensively for decorative carvings, cabochons for jewelry, chess sets, bookends, and interior architectural finishes. Due to its softness, it is mostly used for ornamental objects rather than everyday rings.
Geological facts
Despite being called 'Onyx' in the gemstone trade, it is actually a form of calcite or travertine. True onyx is a variety of chalcedony (silica) with a Mohs hardness of 7. This material is much softer and will react (fizz) if exposed to weak acids like vinegar.
Field identification & locations
Identify in the field by its distinct parallel banding and soft surface that is easily scratched by steel or even a penny. Commonly found in Mexico, Pakistan, and the Southwestern United States. Collectors should look for vibrant green banding and translucency.
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