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 greens, creams, and rust-browns; 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 greens, creams, and rust-browns
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale (can be scratched by a copper coin); Color: Banded greens, creams, and rust-browns; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral; Specific gravity: 2.71.
Formation & geological history
Formed as a chemical sedimentary rock through the precipitation of calcium carbonate from cold spring water or groundwater in caverns (travertine/stalagmitic formation). This material is often relatively young in geological terms, dating from the Quaternary period.
Uses & applications
Primarily used for decorative carvings, ornaments, chess sets, small sculptures, and household items like the chalice pictured. Also used in interior design for tiles and countertops.
Geological facts
Despite being called 'Onyx' in the trade, it is not true onyx (which is a form of chalcedony/silica). Because it is made of calcite, it will react and fizz if it comes into contact with even weak acids like vinegar or lemon juice.
Field identification & locations
Identify in the field by its distinct colorful banding and softness compared to quartz. It is commonly found in Mexico, Pakistan, and Turkey. For collectors, look for translucency and the absence of deep scratches, which are common due to the material's softness.
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