Finds of cobalt blue spinel in Lục Yên, Vietnam

History

The picturesque hills between An Phú and Lục Yên villages in Northern Vietnam hid their wealth of gemstones quite well. Only in 1983 here were discovered fine rubies and around 1987 – spinels as well.

This was the first big find of gemstones in Vietnam. The state-organized cooperation with Thailand in order to learn the mining, processing, and marketing of the newly found wealth.

Large-scale mining ended here around 1994 because the terrain here is complicated and the finds are scattered.

Now the mining is continued by small companies and individual people. For the most part, the work is done by groups of local farmers with their families and friends between the rice farming seasons. Most of the mining is done in alluvial deposits and also in the marble bedrock. Some finds though are inside the natural caves and some are even in rice paddies.

Unusual, neon-blue spinels were discovered here in the early 2000ies and since then this is the best-known find of these gorgeous stones in the world. Occasional spinels of this rare color have been found in Sri Lanka, Madagascar, and Pakistan. Promising is a find (late 2018) in Canada (Baffin Island), as well as beautiful and comparatively stones that are found some 20 km southeast of Mahenge in Tanzania (autumn 2021).

Gemstones of Lục Yên

In the hills are mined rubies, sapphires, spinels, and other valuable stones – occasional tourmaline, nice pieces of feldspar. Recently have been found also yellow danburite crystals.

The best local places to buy the stones are Yên Thế market, An Phu Old Market, Lục Yên markets, and some more markets around the mines.

Spinels of Lục Yên are among the best ones in the world – here often are found intact octahedral crystals. Spinel comes here in the traditional pink and red colors, there happen also interesting sky-blue spinels that change their color to lavender under artificial light. Beautiful are also bright, vivid pink spinels that seem to be glowing.

The most famous find though is a very rare, neon-blue spinel.

The cobalt blue spinel

Local gemstones are mined mainly from An Phu Formation – a nice, white marble that formed in the Upper Proterozoic and Lower Cambrian. This marble has been intruded by granite and pegmatite dikes, it includes also layers of amphibolites. The layer of marble, which bears the spinels, is more than 500 m thick.

While in some lenses of the white marble are found pink and red spinels, in some others have been found the rare blue ones. Research shows that this pure blue color is created by cobalt and also iron. Iron though does not play the main role – it seems, this element makes the hue more grey. Also, if there is too much cobalt, the stone is dark.

The best find of cobalt spinels has been Bai Son mine, where initially (around 2010) the stones were mined from the marble but now – from the gravel and mud around the marble cliffs. Here the stones are up to 5 carats large and have a glowing, bright blue color. Nice stones have been found also in the rice paddies north of An Phú and in some more places.

The value of spinels in the international market is increasing over the last years – and, why not? This is absolutely noble gemstone with similar qualities to the best rubies, sapphires, and emeralds. Cobalt blue spinel could be the most expensive variety.

It is also very hard to fake the color – it comes naturally and even a heat treatment is not needed. Since 2015 has been noticed a technology to color greyish spinels in vivid blue color with so-called cobalt diffusion treatment. An experienced gemologist can tell the difference rather easily.

References

1. Le Thi Thu Huong, Tobias Haeger, The-Long Phan. Between Rice Paddies and Karst Pinnacles. The Luc Yen Gemstone Mining Area, Vietnam Journal of Earth Sciences, 40(1), 47-55, Doi:10.15625/0866-7187/40/1/10915. Page accessed on 16th June 2019.
2. Adrien Trivier. Study of impurity in blue spinel from the Luc Yen mining area, Yen Bai province, Vietnam, InColor, Vol.2 2018 (Issue 39). Page accessed on 16th June 2019.
3. Boris Chauviré, Benjamin Rondeau, Emmanuel Fritsch, Phillipe Ressigeac, and Jean-Luc Devidal. Blue Spinel from the Luc Yen District of Vietnam, Gems & Gemology, Spring 2015, Vol. 51, No. 1. Page accessed on 16th June 2019.

The finds of cobalt spinels in Lục Yên are included in the following article: