Fingerprint-like inclusions are common features in many colored gemstones such as ruby and sapphire, but they are extremely rare in diamonds. In corundum, these “fingerprint” patterns are formed by fluid-assisted partial healing of pre-existing fractures. However, in the case of diamond, much higher pressures and temperatures are necessary to promote partial healing of fractures and, at these conditions, fluids are usually not present. A few instances of fingerprint-like patterns produced by groups of tiny inclusions in natural-color blue and colorless diamonds have been reported, but the interconnected channel-like structure that is common to sapphire “fingerprints” was not observed in these stones (see Lab Notes: Spring 1968, pp. 278–279; Spring 1993, pp. 47–48).

In recent years, fingerprint-like inclusions seen in colorless to near-colorless diamonds are most often associated with high pressure, high temperature (HPHT) treatment (see, e.g., figure 10 in T. M. Moses et al., “Observations on GE-processed diamonds: A photographic record,” Fall 1999 Gems & Gemology, p. 19). Similar to the HPHT-treated stones described by Moses et al., a small “fingerprint” extending from a graphitized inclusion was recently seen in an F-color, 4.79 ct, type IIa heart-shaped brilliant that was found to have been HPHT treated (figure 1).

Over the past few months, the West Coast laboratory has had the opportunity to examine three natural-color diamonds with a range of fingerprint-like inclusions. A pattern consisting of several groups of tiny crystals, very similar to those described in the 1968 and 1993 Lab Note references above, was observed in a Light blue, 0.64 ct, type IIb marquise brilliant. However, the most intriguing discoveries were two colorless type IIa diamonds (a 2.28 ct D-color round brilliant and a 1.00 ct F-color pear shape) that contained inclusions with an appearance remarkably similar to the “fingerprints” seen in rubies and sapphires (figure 2). The diamonds were tested very carefully and determined to be of natural color. The channel-like patterns (not composed of tiny crystals) very strongly suggested that these were partially healed fractures in natural, untreated diamonds.

The geologic environment in which these two diamonds may have been heated to the temperatures necessary to cause partial healing of fractures remains a mystery. The heating must have occurred very deep in the earth (i.e., at high pressures), in that the clarity of these relatively large gem-quality diamonds did not show any evidence of the intense graphitization that occurs in diamonds heated at lower pressures. These samples also serve as a caution to gemologists that fingerprint-like features in colorless or near-colorless diamonds do not always mean the stones have been HPHT treated.

Christopher M. Breeding
GIA Laboratory
Carlsbad, California