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Early this year, the Gem Testing Laboratory (GIT-GTL) in Thailand received two greenish blue faceted stones claimed to be new gemstone “Laurentthomasite” for identification. This mineral was discovered in Madagascar’s Toliara Province, and it was approved by the International Mineralogical Association (IMA) as a new species of milarite (a mineral group) in April 2019. The ideal chemical formula is KMg2AlBe2Si12O30 with hexagonal crystal symmetry. It is named after the French mineral dealer Laurent Thomas of Polychrom, France.
These stones were tested with both standard and advanced gemological instruments. The standard testing revealed the RIs 1.555–1.560 (birefringence 0.005), doubly refractive, uni-axial positive (DR,U+), SG of 2.59–2.63, and inert to long- and short-wave UV radiation. Therefore, the most prominent characteristic was their strong dichroism in greenish-blue and light greenish-yellow.
Microscopic observation showed various inclusion features, such as fingerprints, negative crystals, and two-phase inclusions. Furthermore, the chemical data obtained are consistent with the formula of Laurentthomasite. The Raman spectrum indicates a close association with milarite.
In conclusion, the gemological characteristics we obtained are similar to milarite, however, Laurentthomasite has distinctly different color/pleochroism than milarite, which is colorless to pale yellow or green. The detailed information of this new gemstone has been published in the latest issue of the Journal of Gemmology, vol. 37(2).
