sedimentary
Banded Onyx (Cave Calcite)
Calcite (CaCO3), often called Mexican Onyx or Travertine
Hardness: 3 (Mohs scale); Color: Brown, tan, cream, and white banding; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral; Specific gravity: 2.71
- Hardness
- 3 (Mohs scale)
- Color
- Brown, tan, cream, and white banding
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 (Mohs scale); Color: Brown, tan, cream, and white banding; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral; Specific gravity: 2.71
Formation & geological history
Formed as chemical precipitates from cold or hot spring water or dripping water in caves (speleothems). Layers represent different mineral concentrations and flow rates over hundreds to thousands of years.
Uses & applications
Used extensively for ornamental carvings, bookends, chess sets, decorative tiles, and as a material for lamp bases and 'onyx' jewelry (though it is softer than true agate onyx).
Geological facts
Despite being called 'onyx' in the trade, this is a form of calcium carbonate (calcite), whereas true onyx is a variety of chalcedony (quartz). It reacts and fizzes when exposed to dilute hydrochloric acid.
Field identification & locations
Identify in the field by its distinct parallel banding, high susceptibility to scratching with a knife, and rhombohedral cleavage. Common in Mexico, Pakistan, and the Southwestern US.
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