sedimentary
Banded Onyx (Calcite Marble)
Calcite (CaCO3) - Trap-rock or Cave Onyx
Hardness: 3 on Mohs scale; Color: Multi-colored bands of gold, cream, grey, and black; Luster: Vitreous to waxy; Crystal Structure: Trigonal/Hexagonal; Cleavage: Perfect rhombohedral; Specific Gravity: 2.71
- Hardness
- 3 on Mohs scale
- Color
- Multi-colored bands of gold, cream, grey, and black
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale; Color: Multi-colored bands of gold, cream, grey, and black; Luster: Vitreous to waxy; Crystal Structure: Trigonal/Hexagonal; Cleavage: Perfect rhombohedral; Specific Gravity: 2.71
Formation & geological history
Formed by the precipitation of calcium carbonate from cold water springs or within limestone caves as dripstone. While the limestone bedrock may be millions of years old, these flowstone formations are often Quaternary in age.
Uses & applications
Extensively used for ornamental carvings, interior countertops, decorative tiles, bookends, and low-cost jewelry beads.
Geological facts
Commercial 'Onyx' is often actually a form of banded calcite or travertine. True geological onyx is a variety of chalcedony (quartz), which is much harder (7 on Mohs scale) than this specimen.
Field identification & locations
Identify by its translucent nature when sliced thin and its reaction (fizzing) to weak acids like vinegar. Commonly found in Mexico, Pakistan, Iran, and Turkey. Collectors should look for distinct, vibrant parallel banding.
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