sedimentary
Banded Calcite (Mexican Onyx)
Calcium Carbonate (CaCO3)
Hardness: 3 on Mohs scale; Color: Multi-colored bands (white, brown, cream, orange); Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral; Specific gravity: 2.71
- Hardness
- 3 on Mohs scale
- Color
- Multi-colored bands (white, brown, cream, orange)
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale; Color: Multi-colored bands (white, brown, cream, orange); Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral; Specific gravity: 2.71
Formation & geological history
Formed as chemical precipitates in caves (as travertine) or around hot springs. Calcite layers deposit over time from mineral-rich water, creating the distinct banding patterns.
Uses & applications
Used primarily for ornamental carvings, decorative tiles, jewelry (beads/cabochons), and as a popular specimen for collectors due to its unique patterns.
Geological facts
Although often marketed as 'Onyx', this is actually a variety of calcite or travertine. True onyx is a variety of chalcedony (silica) and is much harder than this material.
Field identification & locations
Identify in the field by its softness (can be scratched by a copper penny or knife) and its effervescence (bubbles) when a drop of dilute acid like vinegar is applied. Commonly found in limestone-rich regions.
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