gemstone
Sapphire
Corundum (Al2O3)
Hardness: 9 on Mohs scale; Color: Blue (primary), also yellow, pink, green; Luster: Vitreous to sub-adamantine; Crystal structure: Trigonal; Specific gravity: 3.95-4.03.
- Hardness
- 9 on Mohs scale
- Color
- Blue (primary), also yellow, pink, green
- Luster
- Vitreous to sub-adamantine
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Physical properties
Hardness: 9 on Mohs scale; Color: Blue (primary), also yellow, pink, green; Luster: Vitreous to sub-adamantine; Crystal structure: Trigonal; Specific gravity: 3.95-4.03.
Formation & geological history
Found in metamorphic rocks like marble and schist, or in basaltic igneous rocks. Large deposits are typically found in alluvial 'gem gravels' where the mineral has weathered out of its host rock.
Uses & applications
Primarily used in jewelry, but also used in industry for non-ornamental windows, infrared optical components, and scratch-resistant watch crystals.
Geological facts
The blue color in sapphire is caused by trace amounts of iron and titanium. Sapphire is the second hardest natural substance on Earth after diamond.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and lack of cleavage. Look for characteristic needle-like inclusions (silk) under magnification.
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