Monday, July 16, 2012

Loving Ethiopian Opals.

For the people who know me, they are also mostly likely to know that I often qualify Ethiopian opals as "the best opals ever"! Is that a biased opinion? Maybe... But for sure, some of them are just exceptional! 
Opal from Wollo, Ethiopia. One of my favorites, it has everything: the play-of-color, the perfection of the structure (pseudo-monocrystal), and the digit pattern. Photo and sample: F. Mazzero, extracted from Rondeau et al. (2011).

What is an opal?


Opal is an amorphous (opal-A) to poorly crystallized (opal-CT) silica that contains water (SiO2, nH20). The water content usually varies from 4 to 10%, but in some cases, it has been reported to be only 0.8% and as high as 21%. It has a specific gravity of about 2, but again, with extreme cases reported down to 0.67 for extremely porous opals (they would actually float!) up to 2.3. Its index of refraction is in between 1.37 and 1.47 with a main value at 1.44. Hardness varies between 5.5 and 6.5, it is therefore quite a soft gemstone (softer than quartz, which has a hardness of 7).
Some opals (either opal-A or -CT) can present what we call play-of-color, which is the fiery rainbow of colors that dances around when moving the stone. This effect is due to the diffraction of light on a network of spheres (for opal-A) or lephispheres (for opal-CT). To learn more about the play-of-color of opals, and why it is, most of the time, absent from opals (the opals are then referred as common opals), you can download here the article from Gaillou et al., (2008).
White, play-of-color opal-A from Tecopa, California. The silica spheres are arranged in a nicely organized network (not the staking faults though!). This opal is very porous, due to the lack of cement in between the spheres.
 White play-of-color opal-CT from Mexico (after HF etching). The network is made out of silica "lephispheres", giving also rise to the play-of-color.


Click to read more about what makes Ethiopian opals special!