In this flattened negative crystal in a Sri Lankan padparadscha sapphire, multiple phases can be found, including both liquid and gaseous carbon dioxide and a diaspore needle. Because diaspore’s refractive index (nα = 1.682–1.706 nβ = 1.705–1.725 nγ = 1.730–1.752) is so close to corundum (1.762–1.770) the diaspore needle almost disappears into the sapphire, appearing like a narrow indentation into the negative crystal. Liquid carbon dioxide becomes a gas at a fairly low temperature, with just the heat of the microscope causing the bubble to disappear. Intact negative crystals such as this are positive proof that the specimen has not been heat treated.
Natural Padparadscha Sapphire • Sri Lanka (Ceylon) • Enhancements: None Detected (None) • Lighting Conditions: Dark Field
Photographer: Richard W. Hughes •
Image Number: A-002-6853-2 • Date Posted: 06 November 2017
Eppler, W.F. (1966) The origin of negative crystals in gemstones. Journal of Gemmology, Vol. 10, No. 2, April, pp. 49–56; RWHL*.
Koivula, J.I. (1980) Carbon dioxide as a fluid inclusion. Gems & Gemology, Vol. 16, No. 12, pp. 386–390; RWHL*.
Hughes, R.W., Manorotkul, W. et al. (2017) Ruby & Sapphire: A Gemologist's Guide. Bangkok, Lotus Publishing, 816 pp.; RWHL*.