sedimentary
Banded Calcite (often sold as Onyx or Mexican Onyx)
Calcite (CaCO3)
Mohs hardness of 3, vitreous to pearly luster, translucent with distinct colorful banding in shades of brown, tan, pink, and white, trigonal crystal system, specific gravity of 2.71
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Physical properties
Mohs hardness of 3, vitreous to pearly luster, translucent with distinct colorful banding in shades of brown, tan, pink, and white, trigonal crystal system, specific gravity of 2.71
Formation & geological history
Formed through the precipitation of calcium carbonate from solution in underground caves, hot springs, or stalactitic deposits over thousands to millions of years
Uses & applications
Commonly carved and polished into decorative objects, sculptures, spheres, lamps, and architectural accents
Geological facts
Often commercially referred to as 'Mexican onyx' or 'cave onyx', though true onyx is a variety of chalcedony quartz. Its softness makes it very easy to carve and polish into intricate shapes.
Field identification & locations
Easily identified by its distinct wavy banding, softness (can be scratched easily with a knife or copper coin), and strong effervescence (fizzing) when exposed to weak acid like vinegar due to its carbonate composition.
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