Rock Identifier
Banded Calcite (often commercially called Mexican Onyx) (Calcite (Calcium Carbonate, CaCO3)) — mineral
mineral

Banded Calcite (often commercially called Mexican Onyx)

Calcite (Calcium Carbonate, CaCO3)

Hardness: 3 on Mohs scale (can be scratched by a copper coin); Color: White, cream, tan, and brown banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal; Cleavage: Perfect rhombohedral.

Hardness
3 on Mohs scale (can be scratched by a copper coin)
Color
White, cream, tan, and brown banding
Luster
Vitreous to waxy
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Physical properties

Hardness: 3 on Mohs scale (can be scratched by a copper coin); Color: White, cream, tan, and brown banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal; Cleavage: Perfect rhombohedral.

Formation & geological history

Formed through the precipitation of calcium carbonate from cold water solutions in caves (forming flowstone) or near springs (travertine). The banding represents different stages of mineral deposition over thousands of years.

Uses & applications

Used primarily for decorative objects, carvings, jewelry (as cabochons), and interior architectural accents like thin wainscoting or lampshades due to its translucency.

Geological facts

Despite being called 'onyx' in the gemstone trade, it is chemically different from true onyx (which is banded chalcedony/silica). It will react and fizz if a drop of weak acid is applied.

Field identification & locations

Identify in the field by its distinct parallel banding, softness compared to quartz, and rhombohedral cleavage. Common locations include Mexico, Utah (USA), and Turkey.

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