sedimentary
Banded Calcite (often sold as 'Mexican Onyx')
Calcite (Calcium Carbonate, CaCO3)
Hardness: 3 on Mohs scale; Color: Cream, tan, brown, and white banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal; Cleavage: Perfect rhombohedral
- Hardness
- 3 on Mohs scale
- Color
- Cream, tan, brown, and white banding
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale; Color: Cream, tan, brown, and white banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal; Cleavage: Perfect rhombohedral
Formation & geological history
Formed through the chemical precipitation of calcium carbonate from cold spring water or hydrothermal solutions, often in caves or around hot springs (travertine/tufa).
Uses & applications
Used primarily for decorative objects, spheres, carvings, bookends, and interior design accents like tiles or counters.
Geological facts
Although frequently marketed as 'Onyx', true onyx is a variety of chalcedony (silica). This material is actually a form of banded calcite or travertine which is much softer than real onyx.
Field identification & locations
Identify in the field by its softness (can be scratched by a copper penny) and its strong effervescent reaction to dilute hydrochloric acid. Common in Mexico, Pakistan, and the USA.
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