sedimentary
Banded Calcite (often sold as 'Mexican Onyx' or 'Green Onyx')
Calcite (CaCO3)
Hardness: 3 on Mohs scale; Color: Translucent green, cream, and brown banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal (microcrystalline in this form); Cleavage: Perfect rhombohedral (though often masked in massive forms); Specific Gravity: 2.71
- Hardness
- 3 on Mohs scale
- Color
- Translucent green, cream, and brown banding
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale; Color: Translucent green, cream, and brown banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal (microcrystalline in this form); Cleavage: Perfect rhombohedral (though often masked in massive forms); Specific Gravity: 2.71
Formation & geological history
Formed through the chemical precipitation of calcium carbonate from cold water solutions, typically in caves (forming stalactites/stalagmites) or near hot springs and limestone fissures. Variations in mineral impurities like iron or copper create the distinct color banding.
Uses & applications
Primarily used for decorative purposes including bowls, chess sets, bookends, and small sculptures. In larger formats, it is used for ornamental architectural tiling and back-lit counter tops due to its translucency.
Geological facts
Despite being commercially marketed as 'Onyx', it is geologically distinct. True onyx is a variety of chalcedony (silica), while this material is calcite (calcium carbonate). It is much softer and more reactive to acids than true onyx.
Field identification & locations
Identify in the field by its distinct parallel banding, waxy translucency, and low hardness (can be scratched by a copper penny). It will effervesce (fizz) if exposed to dilute hydrochloric acid or strong vinegar. Major sources include Mexico, Pakistan, and Turkey.
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