Sapphire

Sapphire is the gem variety of corundum, crystalline aluminum oxide (Al₂O₃), in any color other than red, with blue the classic hue produced by trace iron and titanium. Sapphire encompasses all gem-quality corundum that isn't red (that's ruby). Blue sapphires get their color from a charge transfer between iron (Fe²⁺) and titanium (Ti⁴⁺) ions in the crystal structure - a mechanism completely different from chromium coloring in ruby.

Like ruby, sapphire forms in aluminum-rich, silica-poor environments. The most prized blue sapphires historically came from a single deposit in Kashmir, India, discovered around 1881 at an elevation of 4,500 meters. These Kashmir sapphires have a unique velvety appearance caused by microscopic rutile silk inclusions that scatter light, creating a soft, luminous blue unlike sapphire from any other source. The deposit produced intensively for only a few decades before being largely exhausted.

Sapphire also occurs in alluvial deposits (gravel beds) where it accumulates after weathering from its host rock. Sri Lanka's gem gravels have produced sapphires for over 2,000 years, including the rare padparadscha - a pink-orange sapphire named after the Sinhalese word for lotus blossom.

Sapphire is identified by its exceptional hardness (9), high specific gravity (4.00), and hexagonal crystal habit. Blue sapphire's color is distinctive, but sapphires come in every color except red. The color-change variety appears blue in daylight and purple in incandescent light.

Distinguish from tanzanite (softer at 6.5, different pleochroism), blue spinel (singly refractive, lower SG), and synthetic sapphire (requires lab testing). Natural sapphire typically contains rutile silk (fine needle-like inclusions) and color zoning visible under magnification.