Grandidierite Inclusions in a Sri Lankan Sapphire
Inclusions serve not only as important features for determining a gemstone's origin but also as indicators of the geological environment in which the host crystal formed.
The authors recently acquired a gem-quality sapphire crystal from Sri Lanka (Figure 17) that contained a diverse array of inclusions. Raman spectroscopy identified numerous dark platelets as graphite (Figure 18a) and transparent crystals as phlogopite (Figure 18b). Both of these minerals have been previously identified as inclusions in Sri Lankan sapphires (Gunawardene & Rupasinghe 1986). However, also present were several colourless transparent elongated crystals (Figure 19), which attracted our attention due to their uncommon appearance as inclusions in sapphire. Raman analysis yielded spectra matching grandidierite in the RRUFF database. Their morphology and undisturbed interface with the host sapphire indicated they were primary inclusions, meaning they were trapped during the host sapphire's crystallisation rather than being introduced at a later stage.
Grandidierite is a rare orthorhombic borosilicate mineral with a Mohs hardness of 7 1/2 and the chemical formula (Mg,Fe)Al3(BO3)(SiO4)O2. It typically forms in aluminous, boron-rich rocks that have undergone thermal or regional metamorphism (Anthony et al. 2001). First described in fine gem quality from southern Madagascar (Bruyere et al. 2016), it is prized by collectors for its rarity and attractive blue-green colouration. Grandidierite also occurs in Sri Lanka, New Zealand, Norway, Algeria, Italy, Malawi and Namibia.
While its mineralogical properties and occurrence in primary deposits are well documented, grandidierite has rarely been reported as an inclusion in other minerals. To the authors' knowledge, this is the second recorded instance of grandidierite as an inclusion in corundum (after Hain & Hughes 2019) and, notably, the first reported in a Sri Lankan sapphire. This report contributes to the inventory of minerals documented in gem-quality sapphires from Sri Lanka, and continued investigation of such inclusions may offer valuable insights into the sapphires' geochemical environment and crystallisation conditions, which are not well known since they are typically mined from secondary deposits.
Ziyun Zhang, Yujie Gao (peter.gao@guildgemlab.com) and Wenli LiaoGuild Gem LaboratoriesShenzhen, China
References
Anthony, J.W., Bideaux, R.A., Bladh, K.W. & Nichols, M.C. (eds) 2001. Grandidierite. Handbook of Mineralogy, Mineralogical Society of America, Chantilly, Virginia, USA, https://www.handbookofmineralogy.org/pdfs/grandidierite.pdf, accessed 20 April 2025.
Bruyere, D., Delor, C., Raoul, J., Rakotondranaivo, R., Wille, G., Maubec, N. & Lahfid, A. 2016. A new deposit of gem-quality grandidierite in Madagascar. Gems & Gemology, 52(3), 266-275, https://doi.org/10.5741/gems.52.3.266.
Gunawardene, M. & Rupasinghe, M.S. 1986. The Elahera gem field in central Sri Lanka. Gems & Gemology, 22(2), 80-95, https://doi.org/10.5741/gems.22.2.80.
Hain, M. & Hughes, E.B. 2019. G&G Micro-World: Grandidierite inclusions in sapphires. Gems & Gemology, 55(1), 111-112.
