sedimentary
Banded Calcite (often sold as Orange Onyx or Mexican Onyx)
Calcium Carbonate (CaCO3)
Hardness: 3 on Mohs scale; Color: Rusty orange, cream, and brown bands; Luster: Vitreous to waxy; Crystal Structure: Trigonal; Cleavage: Perfect rhombohedral; Specific Gravity: 2.71
- Hardness
- 3 on Mohs scale
- Color
- Rusty orange, cream, and brown bands
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale; Color: Rusty orange, cream, and brown bands; Luster: Vitreous to waxy; Crystal Structure: Trigonal; Cleavage: Perfect rhombohedral; Specific Gravity: 2.71
Formation & geological history
Formed through the precipitation of calcium carbonate from cold water solutions in caves or around hot springs (travertine/tufa). These layers represent different mineral concentrations and flow rates over time.
Uses & applications
Primarily used for decorative items, sculptures, bookends, and metaphysical crystals. Too soft for most jewelry but common in ornamental carvings.
Geological facts
Despite being called 'Onyx' in the trade, it is actually a form of calcite; true onyx is a variety of chalcedony (silica) and is much harder.
Field identification & locations
Identify by its softness (can be scratched by a copper penny or steel knife) and reaction to dilute hydrochloric acid (it will fizz). Commonly found in Mexico, Pakistan, and Utah.
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