sedimentary
Banded Calcite (often sold as 'Mexican Onyx' or 'Green Onyx')
Calcite (Calcium Carbonate, CaCO3)
Hardness: 3 on Mohs scale (soft); Color: Light green, honey-brown, and cream bands; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral; Specific Gravity: 2.71
- Hardness
- 3 on Mohs scale (soft)
- Color
- Light green, honey-brown, and cream bands
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale (soft); Color: Light green, honey-brown, and cream bands; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral; Specific Gravity: 2.71
Formation & geological history
Formed by the rapid precipitation of calcium carbonate from cold mineral springs or groundwater in caves, creating layered deposits known as travertine or tufa. These deposits can range from thousands to millions of years old.
Uses & applications
Used extensively for ornamental carvings, bowls, chess sets, bookends, and decorative architectural tiles. It is generally too soft for jewelry like rings but popular for pendants and beads.
Geological facts
Despite being called 'Onyx' in the trade, it is not true onyx (which is a form of chalcedony/silica). You can easily distinguish it because it will fizz when in contact with a weak acid like vinegar, whereas true onyx will not.
Field identification & locations
Identify in the field by its distinct parallel banding and extreme softness (it can be scratched by a copper penny or a steel knife). Commonly found in Mexico, Pakistan, Iran, and the Southwestern United States.
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