sedimentary
Banded Onyx (Mexican Onyx / Travertine)
Calcite (CaCO3) - Banded Travertine
Hardness: 3 on Mohs scale; Color: Cream, tan, brown, and translucent white; Luster: Waxy to vitreous when polished; Crystal Structure: Trigonal/Hexagonal; Cleavage: Perfect rhombohedral; Specific Gravity: 2.71
- Hardness
- 3 on Mohs scale
- Color
- Cream, tan, brown, and translucent white
- Luster
- Waxy to vitreous when polished
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Physical properties
Hardness: 3 on Mohs scale; Color: Cream, tan, brown, and translucent white; Luster: Waxy to vitreous when polished; Crystal Structure: Trigonal/Hexagonal; Cleavage: Perfect rhombohedral; Specific Gravity: 2.71
Formation & geological history
Formed through the precipitation of calcium carbonate from mineral springs or groundwater in cave environments (speleothems). This chemical sedimentary process creates characteristic parallel bands of varying colors based on mineral impurities like iron oxides.
Uses & applications
Used extensively for ornamental carvings (like the dolphin shown), chess sets, bookends, interior wall cladding, flooring, and decorative bowls. It is softer than true quartz onyx, making it easier to carve.
Geological facts
Despite being commercially called 'Onyx,' this material is actually a form of marble or travertine. True onyx is a variety of chalcedony (quartz) with a Mohs hardness of 7, whereas this calcite onyx is much softer and will react (fizz) with dilute hydrochloric acid.
Field identification & locations
Identify in the field by its distinctive parallel banding and translucency; it can be scratched easily with a copper penny or steel knife. Common locations include Mexico, Pakistan, Iran, and Turkey. Collectors look for high translucency and vivid color contrast in the bands.
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