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!

Why are pink diamonds pink?

It sounds like a trivial question, right? Well, the answer is no. But first of all, let me introduce you to a few celebrities and the record prices they drew at recent auctions.

CELEBRITIES:

The 24.78 ct Graff Pink diamond sold for $46 million at a Sotherby's auction in 2010. It is the most expensive gem ever sold at an auction. This diamond is a type IIa diamond (no detectable nitrogen impurity by infrared spectroscopy) graded "Fancy Intense Pink" (Sotherby's picture).

The 12.04 ct "Martian Pink" diamond sold for $17.4 million at a Christie's auction in May 2012. Another rare type IIa diamond, graded "Fancy Intense Pink" as well (Christie's picture).

The 8.20 ct "Fancy Intense Purplish-Pink diamond" sold for $2,042,500 at a Christie's auction in 2011 (Christie's picture).

What is all this fuss about pink diamonds? And are they so rare to justify their price?

Well, to be honest, the pink color in diamond is one of the least understood by scientists. I have been conducting research on the subject over the past 5 years, and my colleagues and I came to a better understanding of these diamonds. It looks like the geologic setting has a lot to do with the origin of the color. Pink diamonds don't grow up with a pink color: it is an post-growth process in a rough/turbulent Earth's mantle that diamonds can acquire a pink color.

Here is a quick summary of the research I have been conducted with my colleagues at the Smithsonian Institution and at the Naval Research Lab.

Click to read more!