sedimentary

Banded Calcite (often marketed as Mexican Onyx or Onyx Marble)

Calcite (Calcium Carbonate, CaCO3)

Hardness: 3 on Mohs scale; Color: Pale green, cream, white with mottled banding; Luster: Vitreous to waxy (especially when polished); Crystal Structure: Trigonal; Cleavage: Perfect rhombohedral; Specific Gravity: 2.71.

Hardness: 3 on Mohs scale
Color: Pale green, cream, white with mottled banding
Luster: Vitreous to waxy (especially when polished)

Identified May 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of calcium carbonate from cold spring water or groundwater in caves, creating localized sedimentary deposits. It is often much younger than the surrounding bedrock, typically Cenozoic in age.

Uses & applications

Primarily used for ornamental purposes such as small bowls, figurines, bookends, and decorative tiles. Not suitable for heavy construction due to softness and acid sensitivity.

Geological facts

Despite being sold as 'Onyx,' this material is actually calcite. True onyx is a variety of chalcedony (silica) and is much harder. This specimen shows the typical translucency and pale green hue of 'Green Onyx' decorative ware.

Field identification & locations

Identified by its softness (can be scratched by a copper coin) and violent effervescence if touched with a drop of weak acid (vinegar or HCl). Commonly found in Mexico, Pakistan, and the Southwestern US.

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