sedimentary
Banded Calcite (often commercially sold as 'Mexican Onyx' or 'Honey Onyx')
Calcite (Calcium Carbonate, CaCO3)
Hardness: 3 on the Mohs scale (can be scratched by a copper coin). Color: White, cream, tan, and honey-yellow bands. Luster: Vitreous to waxy. Crystal structure: Trigonal/Hexagonal, but typically seen in massive/banded form. Cleavage: Perfect rhombohedral. Specific gravity: 2.71.
- Hardness
- 3 on the Mohs scale (can be scratched by a copper coin)
- Color
- White, cream, tan, and honey-yellow bands
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on the Mohs scale (can be scratched by a copper coin). Color: White, cream, tan, and honey-yellow bands. Luster: Vitreous to waxy. Crystal structure: Trigonal/Hexagonal, but typically seen in massive/banded form. Cleavage: Perfect rhombohedral. Specific gravity: 2.71.
Formation & geological history
Formed through the chemical precipitation of calcium carbonate from mineral-rich spring water or groundwater in caves (forming stalactites/stalagmites) or along vein deposits. This material typically dates from the Holocene to Pleistocene epochs.
Uses & applications
Used widely for decorative carvings, sculptures, chess sets, and light-filtering lampshades due to its translucency. It is also used in home decor for tiles and tabletops.
Geological facts
Despite being called 'Onyx' in the stone trade, it is not true onyx (which is a form of chalcedony/silica). Because it is made of calcite, it will react and fizz (effervesce) if it comes into contact with a weak acid like vinegar.
Field identification & locations
Identify in the field by its distinct parallel banding, translucency, and relative softness compared to quartz. It is commonly found in Mexico, Pakistan, and the Southwestern United States. Collectors look for high translucency and distinct, colorful layering.
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