sedimentary
Banded Calcite (often sold as Mexican Onyx)
Calcium Carbonate (CaCO3)
Hardness: 3 on Mohs scale; Color: Cream, white, and tan with circular/layered banding; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral.
- Hardness
- 3 on Mohs scale
- Color
- Cream, white, and tan with circular/layered banding
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale; Color: Cream, white, and tan with circular/layered banding; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral.
Formation & geological history
Formed by the precipitation of calcium carbonate from cold water solutions in caves or around warm springs (travertine), often in the Cenozoic era.
Uses & applications
Decorative carvings (eggs, spheres), ornamental architecture, bookends, and holistic gemstone use.
Geological facts
Despite being called 'Onyx' in the trade, it is chemically different from true Onyx, which is a form of chalcedony (silica). Calcite will fizz when in contact with weak acid.
Field identification & locations
Identify in the field by its characteristic banding, softness (can be scratched by a copper penny), and reaction to acid. Often found in Mexico, Pakistan, and the USA.
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