Negative crystals can contain a variety of substances. In this example, on the left we see what are likely diaspore needles, then a graphite flake in the center, and a gas bubble on the right. All show no signs of heat treatment. The gas bubble is the gaseous phase of the liquid carbon dioxide that fills the cavity.
Natural Color-Change Star Sapphire • Sri Lanka (Ceylon) • Enhancements: None Detected (None) • Lighting Conditions: Dark Field
Photographer: E. Billie Hughes •
Image Number: A-002-9387-1 • Date Posted: 30 May 2018
Hughes, R.W., Manorotkul, W. et al. (2017) Ruby & Sapphire: A Gemologist's Guide. Bangkok, Lotus Publishing, 816 pp.; RWHL*.
Eppler, W.F. (1966) The origin of negative crystals in gemstones. Journal of Gemmology, Vol. 10, No. 2, April, pp. 49–56; RWHL*.