Sapphire is a precious gemstone and part of the most important gem family - corundum, which also includes ruby. Both sapphire and ruby have been used in important jewellery, such as imperial crowns, for centuries, and have consistently risen in value and popularity, due to their rarity, beauty, durability and versatility. Sapphire and ruby have the same chemical composition and structure, with presence of certain metals determining their colour. Corundum is also known for its high density, unusual for a transparent mineral.
Sapphire is the most precious blue gemstone, and blue is sapphire's most well known colour. But sapphires naturally occur in a rainbow of colours beside various shades of blue: teal, black, white, grey, orange, pink, green, purple, yellow and the rare and extremely valuable Padparadscha (pinkish orange).
Bi-colour or parti sapphires (stones with two colours in the same crystal, eg yellow and blue) occur in Australia as well as Africa (Madagascar, Tanzania and Nigeria).
All sapphires are made of the same elements, aluminium oxide (α-Al2O3)but the trace elements present in the growing process, such as such as iron, titanium, chromium, vanadium, nickel and magnesium, give each stone its unique colours.
Corundum family is the hardest, most durable natural gemstone after diamond and measures 9 on the Mohs hardness scale, making it a popular choice for jewellery worn every day, such as engagement rings. Despite their durability, they should be protected like any other precious stone, and sapphire jewellery should be removed before engaging in heavy work, such as gardening or construction, or work that would expose them to harsh chemicals.
Sapphires have for centuries been associated with wealth and royalty. Sapphires, especially bigger stones, are valuable gemstones, but those of high colour saturation and particular shades of blue - royal blue and cornflower blue), as well as pinkish orange (Padparadscha) are particularly prized. Blue sapphires are valued for the purity of the blue hue, without secondary hues of purple or green. Blue sapphires of vivid saturation and pure blue hue command the highest prices. Vivid pink sapphires, their colour and saturation determined by the quantity of chromium, are also highly prized. Red corundum is called ruby, less saturated stones are classified as pink sapphires.
Sri Lanka produces many beautiful sapphires of gem quality. Blue sapphires from Kashmir and Sri Lanka are among the most valuable. Other sources of sapphire include Africa (Tanzania, Nigeria, Madagascar, Kenya, Malawi), Brazil, Myanmar (Burma), Thailand, Vietnam and United States (Montana). Many of the unique parti and green sapphires hail from Australia (Queensland and New South Wales).
Heating sapphires is a common treatment that lightens or intensifies colour, improves uniformity and enhances clarity. A sapphire is gently heated in a kiln to remove or dissolve any silky rutile inclusions back into the matrix of the stone. Heating does not damage the sapphire and is a lasting treatment that does not wear off with time.
Sapphire and ruby have been synthesised since the beginning of the 20th century. Synthetic sapphire is mostly used in many industrial applications, such as scratch resistant glass, semiconducting circuits and lasers. Synthetic sapphires can be recognised under magnification by their curved growth lines following the top surface of the rough crystal. Natural corundum crystals have angular growth lines expanding from a single point and following the planar crystal faces.
Hardness: 9 Mohs
Specific Gravity: 3.95-4.03 (sapphire)
Refractive Index: 1.760-1.774
Crystal Form: Trigonal. Sapphire crystals occur as barrel-shaped, double-pointed hexagonal pyramids and tabloid shapes. Corundum is found in igneous and metamorphic rocks and also in alluvial deposits.
Special Care: None
Durability: Very good
Image: Parti sapphire engagement ring by Lizunova Fine Jewels