sedimentary
Banded Calcite (often commercially sold as Mexican Onyx or Green Onyx)
Calcium Carbonate (CaCO3)
Hardness: 3 on Mohs scale; Color: Pale green with cream, white, and rusty brown banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal/Rhombohedral; Specific Gravity: 2.71.
- Hardness
- 3 on Mohs scale
- Color
- Pale green with cream, white, and rusty brown banding
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale; Color: Pale green with cream, white, and rusty brown banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal/Rhombohedral; Specific Gravity: 2.71.
Formation & geological history
Formed in caves (as travertine) or by cold spring water precipitation where mineral-rich water deposits layers of calcite over time. These deposits are typically from the Cenozoic era, though age varies by specific deposit site.
Uses & applications
Primarily used for decorative ornaments, carvings, chalices, chess sets, and architectural accents like lamps or tiles. It is too soft for heavy jewelry use but popular in decorative home goods.
Geological facts
Despite being called 'Onyx' in the trade, it is not true onyx (which is a variety of chalcedony/silica). It is a variety of travertine calcite. One famous historical use includes the carving of small vessels in ancient Egypt and Mesopotamia.
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). Commonly found in Mexico, Pakistan, and Turkey. For collectors, look for translucency and distinct, high-contrast banding.
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