Mineral
Diaspore
AlO(OH)Diaspor
A rare mineral that looks greenish in daylight and pinkish under incandescent light; gem-quality material comes almost only from Muğla.
Also known as: Colour-change diaspore · Zultanite (trade name) · Csarite (trade name)
- Gemstone
- Ore
- Important in Türkiye
In short
Diaspore is a mineral made of aluminium, oxygen and hydrogen. It usually occurs as inconspicuous grains in aluminium-rich rocks such as bauxite. In the İlbir Mountains near Milas in Muğla, however, it forms crystals clear enough to be cut as gems, and their colour changes with the light.
Photos
How does it form?
Diaspore is a mineral of aluminium-rich settings. It is one of the components of bauxite, which forms when rocks are deeply weathered in hot, humid climates, so it is part of aluminium ore. When bauxite is later buried and transformed by heat and pressure, diaspore occurs together with corundum, magnetite and spinel in metamorphic rocks such as emery. The gem diaspore of Milas also formed in old bauxite layers within metamorphic rocks.
Türkiye’s colour-change gem
Transparent, gem-quality diaspore is extremely rare. According to GIA, among known sources such material comes almost exclusively from the İlbir Mountains area in Muğla. In Türkiye these stones are known as sultanit; internationally they are sold under trade names such as “Zultanite” and “Csarite”. Their perfect cleavage makes them hard to cut, and only a small part of the rough is good enough to facet.
What makes it valuable?
The most important factor is how distinct the colour change is: stones that shift clearly from green to pink are the most sought after. Clarity and size matter too; large, clean stones are very rare.
How to recognise it
- 1
Colour change: greenish-yellow (khaki) in daylight, turning pinkish to champagne in candlelight or incandescent light.
- 2
It has a perfect cleavage in one direction, so it is sensitive to knocks and crystals can split into thin plates.
- 3
Its hardness is 6.5–7; its specific gravity (about 3.4) is clearly lower than that of corundum (about 4.0).
- 4
Confirming that a cut stone is diaspore needs laboratory tests such as refractive index measurement and spectroscopy.
Fakes and imitations
What shops usually don’t tell you:
- Colour-change glass: glass containing rare-earth elements such as neodymium and praseodymium has been documented as a diaspore imitation (GIA, 2016).
- Synthetic materials such as laboratory-grown colour-change corundum are widely used to imitate colour-change gems.
- “Zultanite” and “Csarite” are registered trade names. A name on a label does not prove a stone is natural diaspore; ask for a report from an independent gem laboratory.
Did you know?
Its name comes from a Greek word meaning “to scatter”: when heated, it crackles and flies apart into small pieces.
Diaspore also shows different colours when light passes through the crystal in different directions (pleochroism), so the cutter must orient the stone to bring out the best colour.
Where is it found?
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- İlbir Mountains, Milas, Muğla, Türkiye · Major depositPractically the only known source of gem-quality colour-change diaspore
- Gümüşdağ, near Ephesus, Türkiye · Historic sourceDiaspore crystals in an emery deposit
- Kossoi Brod, Ural Mountains, Russia · Historic source
- Chester, Massachusetts, USA · Known localityFormer emery mines
Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →
Sources
- Handbook of Mineralogy: Diaspore · Mineralogical Society of America
- Color-Change Glass as a Zultanite Imitation · GIA, Gems & Gemology, 2016
External links
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