sedimentary
Banded Calcite (often sold as Onyx or Mexican Onyx)
Calcite (Calcium Carbonate, CaCO3)
Hardness: 3 on Mohs scale; Color: White to cream with translucent banding; Luster: Vitreous to pearly; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral
- Hardness
- 3 on Mohs scale
- Color
- White to cream with translucent banding
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 3 on Mohs scale; Color: White to cream with translucent banding; Luster: Vitreous to pearly; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral
Formation & geological history
Formed through the precipitation of calcium carbonate from cold water solutions in caves (as flowstone) or around hot springs (as travertine).
Uses & applications
Predominantly used for decorative carvings, figurines, lamp bases, and ornamental architectural tiles due to its softness and ability to take a high polish.
Geological facts
Despite being called 'Onyx' in the trade, true Onyx is a variety of chalcedony (silica), whereas this material is carbonate-based and much softer. It is often translucent when carved thin.
Field identification & locations
Identify by its relative softness (can be scratched by a copper penny or steel knife) and its reaction to dilute hydrochloric acid (it will fizz). Look for rhythmic banding patterns.
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