Gem-quality diaspore is best known for its colour change, an optical effect that occurs when different types of lighting interact with the gemstone.
In daylight, this stone is usually a greenish colour, that changes to a pinkish brown or even red in incandescent light. Trace elements of titanium, vanadium, iron and chromium within the structure are responsible for this effect.
It is this variety of diaspore, which many may know by the name of Zultanite, that consumers encounter on cruise ships and other tourist markets around the world. This material is not readily available as the market is controlled by two producers.
Aside from colour change, diaspore may also be pink, yellowish, bluish, greenish brown, white, or colourless.
The hue of diaspore tends to vary across deposits around the world – green and purple from Norway, reddish pink from South Africa and chromium-rich green from Russia.
A recently discovered deposit in Pakistan produces a particularly attractive purplish pink that is growing in popularity. The colour change variety is exclusive to the Anatolian Mountains of Turkey.
With a brilliant lustre and very high dispersion, this gemstone breaks up light into the seven spectral colours better than diamond. Such impressive fire contributes to the appeal of a well-cut diaspore.
Crystallising in the orthorhombic system, diaspore is strongly trichroic. This means three different colours are prominent when viewing the specimen at different angles – usually a violet-blue, red, and pale green – an effect distinct from colour change.
This, coupled with the colour change phenomenon and the rarity of transparent gem-quality diaspore, are three main factors that determine its value.
Another feature affecting the value is a rather rare cat’s eye chatoyancy – a captivating single-ray gleam of light reflected off the inclusions within, and best shown when cut en cabochon.
The hardness of diaspore sits at 6.5-7 on Mohs’ scale. Being on the slightly softer side and featuring two planes of cleavage – one perfect and one distinct – it is best suited to less exposed jewellery not intended to be worn every day.
Knocking this gemstone in just the right spot will split it in two!
Navigating this cleavage when cutting diaspore can be very tricky. Large gem- quality specimens are very rare, partly due to the amount of material lost in the fashioning process.
When working with this gemstone, it is important to be aware and cautious of its brittle nature. Warm soapy water is recommended in place of ultrasonic cleaning.
Synthetic diaspore is not available – potentially a result of a lack of substantial demand to drive commercially viable synthesis. Gemstones labelled as synthetic diaspore are likely to be another synthetic material with a colour-change effect imitating Zultanite.
In the world of gemstones, imitation gems are often created and marketed to resemble more expensive or rarer crystals.
In the case of diaspore, colour change glass ‘doped’ with rare earth elements is an example of an imitation.
However, is a relatively easy stone for the trained gemmologist to identify and separate from other material.
Though not as pronounced as alexandrite, the colour change phenomenon is highly desired. This characteristic, combined with an attractive high dispersion and the rarity of diaspore, make it admired and collected by many.
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