Not sapphire! is a vibrant blue spinel that has been colored by cobalt (rather than iron, which may cause the stone to be a darker blue). The most coveted material is from northern Vietnam, although blue spinel is also found in Tanzania, Sri Lanka, Myanmar, Pakistan and Madagascar. Due to cobalt spinel's popularity, many traders apply the name to any spinel that is blue. To be a true cobalt spinel, however, it needs to be colored by enough cobalt and be in the correct color range. Some traders use cobalt
diffusion treatment to increase the intensity of color in blue spinel!
(Image 1) Copyright: Senoble & Bryl (Image 2) Copyright: Lee Groat (Image 3) Photo: Robert Weldon/GIA. Courtesy: Pala International, Fallbrook, California
New spinel treatment discovered involving heat- and cobalt-diffusion.
Cobalt-spinels are considered one of the most precious spinel and are rarely colored blue with or without grey to black tones. Blue Cobalt spinel are at least 10-100x more expensive per carat than grayish-blue colored spinel. Cobalt intensifies the blue color in spinel, considerably enhancing its beauty. Cobalt-spinel over 5ct can be 10,000 to 30,000 US Dollar per carat depending on beauty, while grayish-blue over-dark stones are only 250 to 500 US Dollar per carat depending on size.
Therefore, the incentives for color-enhancement by cobalt diffusion treatment of spinel are high and it should appear sooner or later in the market. Although we have not seen much of this new treatment yet, its appearance in the market is now a reality. Fig. 307a The new type of blue spinel appeared in Sri Lanka in a local market in Beruwala and was brought to our attention by local gemologist MTM Haris.
Fig. 1b Three different Cobalt spinels are shown. On the left are 2 Cobalt spinel from Vietnam and on the right is the new example of a Cobalt-diffused spinel that appeared on the Sri Lankan market.
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