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March 3, 2008
Volume 86, Number 09
p. 64

Science Friction with Bob Wolke

Heather Mull
Bob Wolke

A couple of weeks ago, my wife, Marlene Parrish, came home from a department store's President's Day sale (I wonder which president they meant?) with, among other things, a bracelet set all around with dozens of diamondlike stones that sparkled and glittered more dazzlingly than any $12.95 bracelet had any right to do. Curious, I read the tag. It said, "Made exclusively with high-quality Swarovski crystal."

I know that crystal is a euphemism for lead glass, but as a mere jewelry-challenged male I had never heard of Swarovski, and as a chemist I couldn't wait to find out what these fiery gems were actually made of.

I have always been fascinated by true chemical crystals. Nowhere else in nature do we find perfect geometric shapes with faultlessly straight edges and polished, glassy surfaces. That such precise, macroscopic structures can reflect the spatial relationships among a handful of submicroscopic atoms is truly remarkable.

Like many of my readers, I'm sure, when I was a boy I grew multicolored "gardens" of fragile stalagmites from a variety of salt crystals dropped into a sodium silicate solution. CoCl2 formed blue spires and turrets, while FeCl3 (the fastest performer) made dark yellow ones and stained my hands for days. I also grew big, solid crystals from saturated solutions of alum, CuSO4, and, of course, sugar, which I later ate as rock candy.

Outside chemistry, the word "crystal" has a variety of connotations. New Age wackos ascribe supernatural powers to mineral crystals such as quartz and calcite. To art-glass mavens, "lead crystal" means glass containing anywhere from 18 to 38% PbO. The lead increases not only the glass' density, but its refractive index and dispersion as well. These create exceptional brilliance and refractive colors when the glass is cut into facets.

(By the way, if you are concerned???and it's a legitimate concern???about lead dissolving out of your "crystal" decanters or glasses into your wine or brandy, read my June 7, 2006, Washington Post column at www.washingtonpost.com/wp-dyn/content

But back to the jewelry.

Bohemia-born Daniel Swarovski invented an automatic faceting machine in 1892 and later established a rhinestone production company in Wattens, Austria. Rhinestones, most often made of lead glass, were named after tiny quartz crystals found on the banks of the Rhine River. Swarovski's glass composition is a secret formula reputed to contain 32% lead.

But here's the real secret: Much of Swarovski rhinestones' rainbow scintillations come not from refraction, but from a vacuum-deposited metal film known in the trade as Aurora Borealis, or AB, which Swarovski and other manufacturers have been using since the 1950s. This bottom coating imparts thin-film interference colors to Marlene's bracelet stones, thereby enabling them to rival the diamond's "fire."

Mystery solved.

But what about diamonds? I'll have nothing to do with them. My slogan is, "A diamond is for never." Virtually all the world's diamonds, from the mine to the jewelry store, are rigidly controlled by a privately held consortium operating in 25 mostly economically depressed African countries. It spends $150 million per year in advertising to convince us that diamonds mean love.

Well, I don't buy it, either figuratively or literally. I love my wife as much as the next guy???loves his wife, that is???but a few years ago I bought Marlene a beautiful ring and matching earrings with sparkling, nondiamond stones made of silicon carbide. No, not Carborundum, but moissanite, a transparent crystalline form of SiC named for Henri Moissan, a Nobel Prize-winning (1906) French chemist who found minute crystals of it in fragments of a meteorite.

Moissanite (properly pronounced MWA-san-ite, not MOY-san-ite) is almost as hard as diamond (9.25 on the Mohs scale versus diamond's 10), is less dense (3.22 versus 3.56 g/cm3, so you get a bigger stone per carat), has a higher refractive index (2.65-2.69 versus 2.42), and has a higher dispersion (0.104 versus 0.044). It is therefore every bit as beautiful as diamond and at a fraction of the price. The only ways it can be told from diamond is by its birefringence and its faint ultraviolet fluorescence.


Bob Wolke can be reached at sciencefriction.wolke@gmail.com.

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Chemical & Engineering News
ISSN 0009-2347
Copyright © 2009 American Chemical Society


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