sedimentary
Banded Calcite (often sold as 'Mexican Onyx' or 'Calcite Onyx')
Calcite (Calcium Carbonate, CaCO3)
Hardness: 3.0 on Mohs scale; Color: Multi-colored bands of reddish-brown, cream, and tan; Luster: Vitreous to waxy; Crystal Structure: Trigonal; Cleavage: Perfect rhombohedral; Specific Gravity: 2.71.
- Hardness
- 3
- Color
- Multi-colored bands of reddish-brown, cream, and tan
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3.0 on Mohs scale; Color: Multi-colored bands of reddish-brown, cream, and tan; 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 mineral-rich spring waters or in caves (stalagmitic formation). The banding represents different layers of mineral deposition over time, often containing iron oxide impurities which create the red and orange hues.
Uses & applications
Primarily used for decorative carvings, ornamental figurines, bookends, and interior architectural elements like tiles or countertops. It is a popular material for lapidary work due to its softness and ability to take a high polish.
Geological facts
Despite being called 'Onyx' in the trade, it is not true onyx (which is a form of chalcedony/silica). Calcite will react and fizz if it comes into contact with even weak acids like vinegar, which is a key way to distinguish it from silicate-based stones.
Field identification & locations
Identify in the field by its characteristic banding and its softness; it can be easily scratched with a copper penny or steel knife. Major deposits are found in Mexico, Pakistan, and the Southwestern United States. Collectors should look for vibrant, distinct color contrast in the bands.
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