mineral
Corundum (Sapphire/Ruby in Matrix)
Aluminium oxide (Al2O3)
Hardness: 9 on the Mohs scale. Color: Often dark grey, brown, or blueish in rough form; luster: adamantine to vitreous; crystal structure: hexagonal (trigonal); cleavage: none, but has a characteristic basal parting; specific gravity: 3.95–4.10.
- Hardness
- 9 on the Mohs scale
- Color
- Often dark grey, brown, or blueish in rough form
- Luster
- adamantine to vitreous
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Physical properties
Hardness: 9 on the Mohs scale. Color: Often dark grey, brown, or blueish in rough form; luster: adamantine to vitreous; crystal structure: hexagonal (trigonal); cleavage: none, but has a characteristic basal parting; specific gravity: 3.95–4.10.
Formation & geological history
Formed in silica-poor environments, typically in metamorphic rocks like marble or schist, or igneous rocks like syenite. This specimen likely comes from an alluvial deposit or a weathered metamorphic contact zone.
Uses & applications
Industrial-grade corundum is used as an abrasive (emery). Gem-quality specimens are faceted into sapphires or rubies. Rough specimens like this are primarily for mineral collectors or 'cabbing' for lapidary artists.
Geological facts
Corundum is the second hardest natural mineral after diamond. When it contains chromium, it is called Ruby; all other colors are called Sapphires. It is frequently used in the manufacturing of scratch-resistant watch crystals and optical instruments.
Field identification & locations
Identify in the field by its extreme hardness (it will scratch glass and quartz easily) and its high density (feels heavy for its size). Commonly found in Sri Lanka, Madagascar, Myanmar, and North Carolina, USA. Look for distinct hexagonal crystal outlines in the matrix.
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