Banded Calcite (Mexican Onyx)

Physical Properties

Hardness: 3 on Mohs scale; Color: Yellow, orange, white, and brown banding; Luster: Vitreous to waxy; Crystal structure: Trigonal/Rhombohedral; Cleavage: Perfect rhombohedral; Specific gravity: 2.71

Formation & Geological History

Formed through the precipitation of calcium carbonate from cold spring water or groundwater in caves and crevices. This specimen shows classic sedimentary deposition layers over time.

Uses & Applications

Used primarily for decorative carvings, ornamental stones, bookends, jewelry (beads/cabochons), and as architectural accents like tiles or sink basins.

Geological Facts

While often called 'Onyx' in the trade, it is actually a form of calcite or aragonite; true onyx is a variety of chalcedony (silica) and is much harder than this material.

Field Identification & Locations

Identify in the field by its softness (can be scratched with a copper penny or steel knife) and its reaction to dilute hydrochloric acid (it will fizz/effervesce). Found extensively in Mexico, Pakistan, and the Southwestern US.

Identified on: 4/19/2026

Mode: Standard