mineral
Rough Corundum (likely Sapphire or Common Corundum)
Aluminum oxide (Al2O3)
Hardness: 9 on the Mohs scale. Color: Brownish-grey, blue-grey, or muddy green (opaque). Luster: Adamantine to vitreous, often dull when rough. Crystal structure: Hexagonal (Trigonal). Cleavage: None, but shows rhombohedral parting. Specific gravity: 3.95–4.10.
- Hardness
- 9 on the Mohs scale
- Color
- Brownish-grey, blue-grey, or muddy green (opaque)
- Luster
- Adamantine to vitreous, often dull when rough
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Physical properties
Hardness: 9 on the Mohs scale. Color: Brownish-grey, blue-grey, or muddy green (opaque). Luster: Adamantine to vitreous, often dull when rough. Crystal structure: Hexagonal (Trigonal). Cleavage: None, but shows rhombohedral parting. Specific gravity: 3.95–4.10.
Formation & geological history
Formed in aluminum-rich igneous rocks (syenites, pegmatites) and metamorphic rocks (mica schists, gneisses, or recrystallized limestones). Often found in alluvial placer deposits due to its extreme hardness and resistance to weathering. Geological age varies by deposit, ranging from Proterozoic to Cenozoic.
Uses & applications
Opaque specimens are primarily used as industrial abrasives (emery). Transparent or vibrantly colored specimens are used as gemstones (Sapphire/Ruby). Synthetic versions are used for scratch-resistant watch crystals and laser components.
Geological facts
Corundum is the second hardest natural mineral after diamond. The color variations are caused by trace impurities; iron and titanium produce blue, while chromium produces red (ruby). The specimen in the image shows distinct 'striations' or growth lines typical of the hexagonal crystal system.
Field identification & locations
Identify in the field by its extreme hardness (it will scratch glass, quartz, and topaz easily) and its high density (feels heavy for its size). Look for hexagonal barrel-shaped crystals. Common locations include Sri Lanka, Madagascar, Myanmar, and North Carolina (USA).
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