Sapphire (Corundum)

Hardness: 9 on Mohs scale; Color: Most commonly blue, but can be found in almost all colors except red (which would make it a ruby); Luster: Vitreous to adamantine; Crystal structure: Hexagonal (trigonal); Specific Gravity: 3.95-4.10; Cleavage: None (has parting)

Sapphires form in metamorphic rocks (e.g., gneiss, schist) and igneous rocks (e.g., basalt, syenite), particularly those rich in aluminum and poor in silica. They crystallize from magma or during high-grade metamorphism. Alluvial deposits (placer deposits) are also a significant source, where sapphires, due to their hardness and density, accumulate in riverbeds after being weathered out of their primary host rocks. Geological age varies greatly depending on the deposit, ranging from hundreds of millions of years to much younger formations.

Prized as a gemstone for jewelry (rings, necklaces, earrings). Industrial uses include abrasives (due to its hardness), wear-resistant components, optical components (e.g., in lasers, spacecraft windows, and certain high-durability watch crystals), and as substrates for electronic circuits.

Sapphire is one of the two gem varieties of the mineral corundum, the other being ruby (red corundum). The blue color in sapphire is primarily due to the presence of titanium and iron impurities. Other colors are due to other trace elements. Star sapphires exhibit a phenomenon called asterism, a star-like effect caused by oriented needle-like inclusions of rutile. It is the birthstone for September and is associated with wisdom, royalty, and prophetic visions.

Due to the small size and potentially water-worn appearance, field identification without further tests can be challenging. However, key indicators for sapphire would be its exceptional hardness (it will scratch glass and most other common minerals), its high specific gravity (it feels denser than similarly sized common pebbles), and its vitreous to adamantine luster. The specimen in the image appears translucent to opaque with a pale blue to gray hue, possibly indicating a lower-grade or raw sapphire, or even a different, harder blue-grey mineral like kyanite or spinel. Always verify hardness. Commonly found in placer deposits in Sri Lanka, Australia, Myanmar (Burma), Madagascar, Thailand, and United States (Montana).