sedimentary
Banded Calcite (often sold as 'Green Onyx')
Calcite (Calcium Carbonate, CaCO3)
Hardness: 3 on Mohs scale; Color: Pale green, white, and cream banding with occasional orange/brown iron staining; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral.
- Hardness
- 3 on Mohs scale
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale; Color: Pale green, white, and cream banding with occasional orange/brown iron staining; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral.
Formation & geological history
Formed through the precipitation of calcium carbonate from cold water solutions in caves or spring deposits (travertine). The banding represents different layers of mineral deposition over time.
Uses & applications
Primarily used for decorative objects, sculptures, bookends, jewelry, and metaphysical purposes. Due to its softness, it is rarely used in industrial construction.
Geological facts
Despite being marketed as 'Green Onyx' in the gem trade, it is geologically a banded calcite. True onyx is a variety of chalcedony (quartz) and is much harder (Mohs 7) than this material.
Field identification & locations
Identify in the field by its characteristic parallel banding and its reaction to dilute hydrochloric acid (it will fizz). It can be easily scratched by a copper penny or a steel knife, unlike quartz-based stones.
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