Banded Calcite (often sold as 'Mexican Onyx')

Physical Properties

Hardness: 3 on Mohs scale; Color: Cream, honey, brown, and green banding; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral. It is translucent and reacts with dilute hydrochloric acid.

Formation & Geological History

Formed by the precipitation of calcium carbonate from cold water solutions in caves (as flowstone or stalagmites) or from springs (travertine). It is a chemical sedimentary rock often associated with limestone regions.

Uses & Applications

Primarily used for decorative purposes including carved ornaments, bowls, bookends, and jewelry. It is also used as a specialty architectural stone for countertops or tiles, though it is soft and susceptible to scratching and acid damage.

Geological Facts

Despite being commonly marketed as 'Onyx', this material is actually a form of calcite or travertine. True onyx is a variety of chalcedony (silica) with a Mohs hardness of 7, making it much harder than this specimen.

Field Identification & Locations

Field identification can be done using a hardness test (it is easily scratched by a copper penny or steel knife) or an acid test (it will fizz when in contact with vinegar or acid). Common locations include Mexico, Pakistan, and various limestone-rich regions worldwide.

Identified on: 5/15/2026

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