Sapphire is part of the most important gem family, corundum, which also includes ruby. Both sapphire and ruby have consistently risen in value and popularity, due to their beauty, durability and versatility.
Sapphires naturally occur in a rainbow of colours: blue, teal, black, white (colourless), grey, orange, pink, green, purple and yellow.
The colour in Australian green sapphire is typically made up of a mixture of yellow and blue coloured banding within the crystal mixing to produce the green colour. When some of these yellow and blue banded sapphire crystals are cut suitably, stones know as ‘parti’ (short for partition) can be created.
Bi-colour or parti sapphires occur in Australia as well as Africa (Madagascar, Tanzania and Nigeria).
A parti sapphire is made of the same elements as other corundum (Al203) but the trace elements present in the growing process give each stone its unique colours. Presence of iron gives parti sapphire its yellow colour, and blue comes from a mix of titanium and iron.
Australian sapphires are from alkali-basalt related deposits and are very rich in iron content. Due to varying concentrations of transition elements (e.g. iron and titanium) in the chemical make-up of the parent fluids (sapphire is essentially aluminium oxide), different colours result. The parti sapphires are so special as each stone has a unique combination of zoning and banding. The colour zoning seen in parti sapphires relates to the growth layers of a crystal, and appears as a series of concentric hexagons parallel to the prismatic crystal faces.
Corundum family, which parti sapphire is a member of, is the hardest, most durable gemstone type 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.
The rarity of parti sapphires makes them valuable, especially bigger stones, and stones with brighter colours. Compared to blue sapphire though, parti sapphire is exceptionally good value.
This remarkable 5.5ct cushion cut parti sapphire displays shades of green, teal and blue. Parti sapphires of this size and colour are very rare. Image: Lizunova Fine Jewels
While sapphires are most commonly sourced from Africa (Tanzania, Nigeria, Madagascar, Kenya, Malawi), Brazil, Myanmar (Burma), Pakistan (Kashmir), Sri Lanka, Thailand and United States (Montana), many of the unique parti and green sapphires hail from Australia (Queensland and New South Wales).
Sapphires were widely mined in Australia in the 1970s. Today, Australian sapphire mining is much smaller in scale, and is most commonly individual miners working on their claim. The cost and complications of sapphire mining have ensured good quality Australian sapphires are much more scarce.
This oval cut Australian parti sapphire displays the classic yellow and blue colours, mixing into green tones at certain angles. Image:Lizunova Fine Jewels
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.
Parti sapphire is the only stone that can’t be synthesised in a lab, due to the patchy colour pattern.
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: Lizunova Fine Jewels
Sources: The Jeweller’s Directory of Gemstones,Judith Crowe; OAGems.com