Sapphires come in almost every colour except red
Blue is only the famous version. Sapphire is corundum, crystallised aluminium oxide, and trace elements tint it yellow, green, purple, pink, orange, black or leave it clear. Red corundum gets a different name, ruby. Lab-grown sapphire, meanwhile, protects watch faces and serves as the base for blue LEDs.
Pure corundum has no colour at all. Tiny amounts of other elements, along with the way the crystal grew and the microscopic inclusions it trapped, create the palette. Where pink sapphire ends and ruby begins depends on how the stone looks and on which trade convention is applied. Some stones show optical tricks: star sapphires display asterism, and others shift colour under different lighting. With a Mohs hardness of 9 they resist scratching superbly.
Corundum forms where aluminium is abundant but silica is scarce, so the aluminium cannot all be locked into ordinary silicate minerals. Metamorphic, magmatic and metasomatic processes can all produce it, and one deposit may record several in sequence. At Dusi in central Kenya, corundum crystallised early from a magma. At the historic Paddar deposits in India, sapphire grew in pegmatite bodies inside altered ultramafic rock, as chemical exchange drained silica along their contacts. Crystals may stay in their parent rock, ride up in a younger magma or weather out into sediments, and miners work both bedrock and these secondary deposits.
Gem value rests chiefly on colour, then clarity, cut, size, treatment history and sometimes origin. Heating to improve colour and transparency is routine, and diffusion treatments can introduce elements from outside, so detecting and disclosing these steps is central to modern gemology.
Factory-grown sapphire took off early in the 20th century, once the Verneuil method made it economical. Its hardness, resistance to abrasion and heat, transparency across a wide range of light and electrical insulation make it valuable well beyond jewellery: watch crystals, protective windows and scientific instruments. Sapphire wafers underpin silicon-on-sapphire electronics and gallium nitride devices such as blue LEDs, and titanium-doped crystals drive titanium-sapphire lasers.
Source: Sapphire
