A historic blue-gray diamond will go on sale at Christie's London on December 10. The diamond, a 35.56 carat beauty, is known as the Wittelsbach diamond and as the Telegraph reports, it was given to Infanta Margarita Teresa by her father King Philip IV of Spain on the occasion of her engagement (to her uncle Leopold I of Austria no less). It was part of first the Austrian and then the Bavarian crown jewels before ending up back in private hands. It has been part of the same collection since 1964.

The diamond has the ideal combination of rarity (blue diamonds of this size are extraordinary scarce) and historical provenance and has been estimated to bring as much as £10 million at Christie's. Even six months ago it would seem a slam dunk that this gem would meet or even exceed estimates (in May a 13.39 carat blue diamond stunner sold for a world record price of $8.9 million). Now, however, even those at the highest ends of the market are cautious.

Overall, the jewelry market has seen signs of weakness lately but the tip of the market is still strong. The blue diamond shown above is a 13.39 carat stunner which sold for a world record price of $8.9 million at a sale at Christie's in Geneva last Wednesday. Blue diamonds of this size are rather rare. The stone is a type IIb diamond. Type IIb diamonds are prized by jewelry lovers and collectors and contain small amounts of boron which gives them their color. They also have the property of being semi-conductors that conduct electricity. The auction brought in nearly $48.3 million total.

[via JCK Online]

The world's rarest gem is believed to be painite, a gem that most have never even heard of. Rarity does not equate to beauty in gems, however. The painite is an orangish or reddish brown, with the brown tint coming from iron in the crystal. It was first discovered in Burma in the 1950s and was widely considered to be the rarest of all gems, with only two faceted crystals in existence. Within the last couple of years, however, the source of the original painite crystals was discovered, and now a few hundred faceted stones exist.

A more widely recognized (but still very rare) gem is the red diamond. There are many more red diamonds available than some other rare gems, but very large red diamonds are extremely rare. The largest cut natural red diamond may be the one owned by Moussaieff Jewellers Ltd, a 5.11 carat trillion cut.

The rare blue diamond, made famous by the Hope Diamond, gets its deep blue color from traces of boron in the stone. It is a natural blue diamond, in contrast with most of today's blue diamonds, which are turned from clear to blue with a process involving irradiation and heat.

The Hope Diamond was believed to be originally over 112 carats, but was poorly cut. It was recut in the 1600's, resulting in a stone just over 67 carats. It changed hands many times, and got its name from Henry Philip Hope, who acquired the diamond in 1839. After it changed hands a few more times, the Hope Diamond came to rest in the care of the Smithsonian, and now weighs over 45 carats.