sedimentary
Banded Calcite (often sold as Onyx Marble)
Calcite (Calcium Carbonate, CaCO3)
Hardness: 3 on Mohs scale (soft); Color: Cream, white, and honey-yellow bands; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral. It will react and bubble (effervesce) if exposed to weak acid like vinegar.
- Hardness
- 3 on Mohs scale (soft)
- Color
- Cream, white, and honey-yellow bands
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale (soft); Color: Cream, white, and honey-yellow bands; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral. It will react and bubble (effervesce) if exposed to weak acid like vinegar.
Formation & geological history
Formed by the precipitation of calcium carbonate from spring water or groundwater, often in caves or around hot springs, creating layers of travertine or flowstone over hundreds to thousands of years.
Uses & applications
Primarily used for decorative items like spheres, bookends, and ornaments. In architectural history, it was used for vases and thin window panels (alabaster-style).
Geological facts
Though widely marketed in the gemstone trade as 'Mexican Onyx' or 'Honey Onyx', it is not a true onyx (which is silicate-based); it is a form of banded calcite that is much softer and more easily carved.
Field identification & locations
Identify in the field by its distinct parallel banding and its softness relative to quartz-based stones (it can be scratched by a copper coin or a steel knife). Common locations include Mexico, Pakistan, and the Southwestern United States.
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