mineral
Banded Calcite (Mexican Onyx)
Calcium Carbonate (CaCO3)
Hardness: 3 (Mohs scale), Color: tan, brown, white, and cream bands, Luster: Vitreous to waxy, Crystal Structure: Trigonal/Hexagonal, Cleavage: Perfect rhombohedral
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Physical properties
Hardness: 3 (Mohs scale), Color: tan, brown, white, and cream bands, Luster: Vitreous to waxy, Crystal Structure: Trigonal/Hexagonal, Cleavage: Perfect rhombohedral
Formation & geological history
Formed as chemical precipitates in caves (travertine) or from hot springs. This banding occurs as water chemistry and flow rate fluctuate over hundreds to thousands of years.
Uses & applications
Used widely in ornamental carvings, chess sets, jewelry, bookends, and decorative architectural elements. It is often sold as 'Onyx' though it is mineralogically calcite.
Geological facts
Despite being called 'Mexico Onyx' or 'Cave Onyx,' it is not true onyx (which is a form of chalcedony/silica). It will react and fizz if a drop of cold dilute hydrochloric acid is applied.
Field identification & locations
Identify by its softness (can be scratched by a copper penny or steel knife) and its characteristic parallel banding. Commonly found in limestone caves and thermal spring deposits worldwide.
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Mineral
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Mineral/Rock
Banded Gneiss (River Rock/Pebble)
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