mineral

Sapphire

Corundum (Al2O3)

Hardness: 9 on the Mohs scale; Color: Deep blue to dark indigo (common in lower grade commercial jewelry); Luster: Vitreous; Crystal structure: Trigonal/Hexagonal; Cleavage: None; Specific gravity: 3.95 - 4.10.

Hardness: 9 on the Mohs scale
Luster: Vitreous

Identified March 10, 2026More mineral →

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Physical properties

Formation & geological history

Formed in metamorphic or igneous environments, specifically high-alumina, silica-poor rocks like marble or basaltic flows. Found in gravel deposits (alluvial) or primary host rocks.

Uses & applications

Primary use is in high-end and commercial jewelry. Industrial uses include scratch-resistant windows, watches, and abrasive components due to extreme hardness.

Geological facts

Sapphire is the second hardest natural substance after diamond. While famous for blue, it occurs in every color except red (which is classified as Ruby). Many dark blue semi-opaque sapphires are heat-treated to improve clarity.

Field identification & locations

Identify by extreme hardness (scratches glass/topaz) and high density. Commonly found in Sri Lanka, Madagascar, Thailand, and Australia. In jewelry like the bracelet shown, verify by looking for silk inclusions under magnification.