sedimentary
Banded Calcite (often sold as 'Travertine' or 'Mexican Onyx')
Calcium Carbonate (CaCO3)
Hardness: 3 on Mohs scale (can be scratched by a copper penny); Color: Tan, beige, brown, and cream; Luster: Vitreous to pearly; Crystal structure: Trigonal/Rhombohedral; Cleavage: Perfect rhombohedral; Specific gravity: 2.71.
- Hardness
- 3 on Mohs scale (can be scratched by a copper penny)
- Color
- Tan, beige, brown, and cream
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 3 on Mohs scale (can be scratched by a copper penny); Color: Tan, beige, brown, and cream; Luster: Vitreous to pearly; Crystal structure: Trigonal/Rhombohedral; Cleavage: Perfect rhombohedral; Specific gravity: 2.71.
Formation & geological history
Formed by the chemical precipitation of calcium carbonate from mineral-rich waters, typically within caves (as flowstone) or in hot spring terraces. This specific specimen shows 'boxwork' or 'ribbed' differential weathering, suggesting it was deposited in layers over thousands of years during the Quaternary period.
Uses & applications
Used primarily for ornamental carvings, bookends, paperweights, and architectural tiling. Though soft, it is popular in 'lapidary' hobbies for polishing.
Geological facts
Despite being called 'Onyx' in the trade, it is not true onyx (which is a form of chalcedony/silica). This material will vigorously effervesce (fizz) if touched with a drop of cold dilute hydrochloric acid or white vinegar.
Field identification & locations
Identify in the field by its distinct parallel banding (lamination) and extreme softness compared to quartz. It is commonly found in limestone-rich regions like Mexico, Turkey, and the Southwestern United States. Collectors look for high translucency and rhythmic color bands.
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