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The skinny on some of the world's priciest gems

By Mary Johnson
February 22, 2009

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Mary Johnson has a doctorate in mineralogy and crystallography and is a former research and development manager at the Gemological Institute of America. Got gemological questions? E-mail them to national jeweler100@yahoo.com

Because certain colored gemstones are extraordinarily beautiful and extremely rare, they command high prices on the global market. A reader recently asked several questions about which colored gemstones carry the highest price tags.

Question: What is the most expensive gemstone in the world, besides diamond: Is it alexandrite, tanzanite or Brazil's Paraiba tourmaline?

Answer: Keep in mind that the top price paid for a gem does not mean that all samples of the same material will be worth as much. Prices depend on size, color, clarity, rarity, fashion, origin, cachet and whether or not a stone is treated. Increasingly, important gemstones only sell with a report from a reputable third-party laboratory that provides an expert opinion on the stone.

The most expensive per-carat colored stone sold recently was an untreated 8.62-carat Burmese ruby mounted in a ring, which sold at auction in 2006 for $3.6 million, or $425,000 per carat. That ruby had an SSEF [Swiss Gemological Institute] report. Another large ruby sold for $275,000 per carat (source: International Colored Gemstone Association*). A 22-carat blue sapphire from Kashmir and a 10.11-carat Colombian emerald have also sold for more than $100,000 per carat. I recently saw prices mentioned for "real" (Brazilian) Paraiba tourmalines as being up to $20,000 per carat.

In the fine qualities of commercially available goods, 1-carat tanzanites sell for about $800 per carat, while 1-carat Mogok rubies can go for $30,000 per carat and fine Colombian emeralds can go for $10,000 per carat (source: National Gemstone **). I couldn't find any recent prices for alexandrites, although they would probably be pricier than tanzanites.  But again, all the factors mentioned above matter.

Question: Is a Burmese ruby or a Colombian emerald necessarily more expensive than same-name stones from other countries?

Answer: A ruby from a classic Burmese source such as Mogok (but not from the less impressive deposits at Mong Hsu), with a report from a reputable laboratory attesting to its source area and lack of treatment, is probably worth more than an identical ruby with a report saying it comes from somewhere else. That's the cachet factor. But a low-quality ruby from a famous locality will not be worth much more than a similar ruby from somewhere else. The same holds true for Colombian emeralds.

However, keep in mind that it is no longer legal to import rubies from Myanmar (Burma) into the United States.

Question: What is Brazil-law twinning in quartz?

Answer: A crystal is an organized structure made of atoms. The quartz crystal structure at high temperature (beta-quartz) is symmetric, but the room-temperature quartz structure is slightly crumpled so that it has lost some symmetry. This means that if you placed a quartz crystal in front of a mirror and super-imposed the mirror image with the original, the atoms would not coincide. The low-temperature quartz structure is said to have chirality, or "handedness." Atoms in regions that are distant from each other may not "know" what handedness the other region has "chosen," so quartz crystals that started out at high temperature can end up with regions of different handedness. That causes Brazil-law twinning.

Editor's note: This story first appeared in the February 2009 print edition of National Jeweler.
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