Diaspore
Diaspore is an aluminium oxide hydroxide mineral with the chemical formula a-AlO(OH). It often occurs in lamellar form (thin plates or scales) but sometimes also forms crystals such as the ones shown in the images.
These crystals as sometimes used as gemstones - however much of the diaspore found is not of gem quality. Diaspore is a component of bauxite - an aluminium ore used industrially to obtain aluminium. Diaspore has also been called empholite, kayserite or tanatarite. [1]
Another set of gemstones that are formed from an aluminium oxide is the corundum stones - sapphire and ruby, which are composed of Al2O3. However, these stones are quite different from diaspore.
Diaspore is isomorphous with goethite - meaning that it has the same or fundamentally the same crystal shape. It also has the interesting quality of making crystal "twins" that are heart-shaped or "fishtail" shaped in cross section. Diaspore has perfect cleavage in one direction - and it is said that this is a major reason why it is one of the most difficult gemstones to facet. [2]
Diaspore also sometimes has the unusual quality of exhibiting a color change effect under different forms of light. The stone itself can occur in colors amber, white, brown, colourless, pale yellow, greyish, lilac, violet, pinkish [1] [3] - however under varied light conditions it can show color change between the green and red spectra. The color change characteristic is stronger in larger stones. A similar color change phenomenon can also be seen in the stone alexandrite.
As with alexandrite, the diaspore color change is considered a highly desirable gemstone quality which can lead to greatly increased prices at retail. Diaspores with good clarity, color and color change characteristic can command high cost: Looking around online (August 2010), I just saw a 4.68 carat diaspore priced at $4680 ($1000 per carat.) However, smaller stones with lesser or no color change can be seen at much lower prices, and I have seen a 1.99 carat stone priced at $70.
I have read that care should be taken when buying color-change diaspore online; as it has been known for photos to be altered to show color change. [2]
The world's main source for gem quality diaspore is Turkey. [2] The stone has also been found in several other locations worldwide - including in Antarctica, Australia, Austria, Brazil, Bulgaria, Canada, Chile, China, Czech Republic, France, Greece and Japan. [3]
Diaspore Images
Diaspore
From Mugla Province, Aegean Region, Turkey. Size 1.8 x 0.6 x 0.4 cm
Photo by Rob Lavinsky, iRocks.com - image lic. under CC-BY-SA-3.0
Diaspore
(From Turkey.)
Image © supplied by Woodmansee* Gems
Diaspore
(From Turkey. Weight: 0.55 carats total for the three stones)
Image © supplied by Woodmansee* Gems
Sources Referenced:
[1] http://en.wikipedia.org/wiki/Diaspore
[2] http://www.atggems.com/Photos_Diaspore.htm
[3] http://www.mindat.org/min-1285.html
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