sedimentary
Banded Calcite (often sold as Onyx)
Calcite (Calcium Carbonate, CaCO3)
Hardness: 3 on Mohs scale (can be scratched by a copper penny or knife); Color: White, cream, brown, tan with distinct banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal (often massive/microcrystalline in bands); Cleavage: Perfect rhombohedral (though often obscured in banded samples);…
- Color
- White, cream, brown, tan with distinct banding
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale (can be scratched by a copper penny or knife); Color: White, cream, brown, tan with distinct banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal (often massive/microcrystalline in bands); Cleavage: Perfect rhombohedral (though often obscured in banded samples); Specific Gravity: 2.71
Formation & geological history
Formed through the chemical precipitation of calcium carbonate from cold spring water or hydrothermal solutions, often in caves (as flowstone/travertine) or around hot springs. Geological age varies greatly from Recent to various Paleozoic eras depending on local strata.
Uses & applications
Used extensively in interior decoration for carvings, bookends, tiles, and countertops. In jewelry, it is used for cabochons or beads, though limited by its softness. Also used as a flux in steel making and for architectural trim.
Geological facts
Commonly mislabeled in the commercial trade as 'Mexican Onyx' or 'Pakistan Onyx,' it is actually a form of calcite or travertine rather than true onyx (which is a form of chalcedony/silica). It will fizz or effervesce if a drop of weak acid (like vinegar) is applied to its surface.
Field identification & locations
Identify in the field by its distinct parallel banding, relative softness compared to quartz (it cannot scratch glass), and its reaction to acid. Common locations include Mexico, Pakistan, Turkey, and various cave-rich regions of the United States like Arizona and Utah. Collectors look for high translucency and vivid color contrasts.
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