Cobalt Spinel

Posted on August 31, 2022 by Çigdem Lüle, PhD, FGA, GIA GG, DGA

Vivid cobalt spinels are particularly rare and more expensive than the other blue spinels. Recently, a new source outside Mahenge in Tanzania has excited the gem trade. MARKET TRENDS Gem quality spinels occur in a range of hues with reds, pinks to lavenders, and blues being particularly important to the trade. Mogok, Burma is known to be the oldest source of pink and red spinel. Namya (Namyazeik or Nanyazeik, near Kachin State) is another, newer source in Myanmar. Other notable sources include Tajik- istan, Vietnam, Tanzania—especially Mahenge, Sri Lanka and Madagascar. Blue spinels are found in Sri Lanka, Tanzania, Myanmar, Pakistan, and Vietnam. Luc Yen and Quy Chau are the two major blue spinel sources in Vietnam. Both sources were discovered at the end of the 1980s and Luc Yen has been a significant source for vivid blue spinels since the 1990s. In the last few years, a new finding of blue and cobalt spinel from Tanzania has excited the trade. Since the cobalt spinels are particularly rare and more expensive than the other blue spinels, this new source outside Mahenge in Tanzania attracted quite an interest. Today, well-saturated blue spinels are very popular, not only due to desirability of blue stones but also increased under- standing of cobalt spinel’s rarity in general. Blue coloration in spinel is created by cobalt (Co2+) substituting for magnesium (Mg2+) in structure. Iron (Fe2+) is present and as the amount of iron increases, the saturation of blue decreases to blue-gray colors. Since iron is more common than cobalt as a coloring agent, vivid cobalt blue spinels are the rarest. Vibrant cobalt blue commands several times more per carat price compared to other blue, grayish blue to lavender spinels. According to Christopher Smith, President of AGL New York, all blue spinels contain cobalt, yet in order to be the rarer cobalt spinel, the iron content should not exceed a threshold. Therefore, the laboratory sets boundaries what should be called blue or cobalt spinel. Smith points out that this new source seems to be rich with both varieties and there are really nice examples. Gemologists know that synthetic blue spinel has been produced in many tones of blue for decades to imitate aquamarine, blue topaz, and blue sapphire, which are easily separated by refractive index. Also, flux synthetic spinel samples masquerade as natural but separable via microscopy. As for treatments, blue spinels have not been reported for any major treatments so far. However, fracture filling should never be dismissed as more and more stones are detected with various fillers in the market, regardless of their identity and quality. Although the cobalt blue spinels are very desirable and demand a premium over blue spinel per carat prices, price stability has been observed for a while in the US market. This trend is contrary to that observed in the pink and red spinels, which have recorded strong price increases during the past decade. Blue spinel prices published are for the more common blue spinel variety. The color can range from pastel to dark blue, greenish blue or violetish blue color. For centuries, blue spinel has been mistaken for sapphire. Historically, prices for fine blue spinel have been relatively affordable com- pared to fine blue sapphire. However, there is of course the exception of the rare cobalt blue material. The GemGuide blue spinel charts require a premium adjustment if used for pricing cobalt spinel and in certain qualities of cobalt spinel, like the exceptional Sri Lankan cobalt spinel that was seen at this year’s JCK Vegas Show priced at more than $100,000 per ct, pricing will bear no relationship to published guidelines for blue spinel. The finest color of cobalt spinel is difficult to describe; it’s one of those gems that you just know how special it is as soon as you see it.

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