Fingerprints are simply healed fissures composed of a pattern of tiny negative crystals. In this case, we can see negative crystals that contain tiny diaspore needles, along with flat hexagonal plates of graphite. Heat treatment will damage these negative crystals and so when intact, they offer proof that the gem has not been heat treated.
Natural Ruby • Sri Lanka (Ceylon) • Enhancements: None Detected (None) • Lighting Conditions: Dark Field
Photographer: Richard W. Hughes •
Image Number: A-002-4792-1 • Date Posted: 27 June 2017
Eppler, W.F. (1959) The origin of healing fissures in gemstones. Journal of Gemmology, Vol. 7, No. 2, April, pp. 40–66; RWHL*.
Roedder, E. (1962) Ancient fluids in crystals. Scientific American, Vol. 207, pp. 38–47; RWHL*.