Investigating artificially coloured diamonds

Abstract

COLOURLESS diamonds are rare—most have a slight natural yellow colour which makes them unsuitable as gem stones. However, a strongly coloured gem quality diamond is very valuable. Neutron irradiation is sometimes used to ‘treat’ diamonds to strengthen and improve the yellow colour, but their value is decreased if the coloration is known to be artificial. For many years gem testing laboratories have concentrated on an absorption line at 595 nm to decide the pedigree of yellow coloured diamonds. Crowningshield¹ examined a very large number of treated and untreated diamonds and found that all treated diamonds except one showed the 595 nm line, whereas in only two cases of uncertain history was this line present in natural untreated stones. The rarity of this line in untreated diamonds has been confirmed by Anderson² who further states (personal communication) that it is never seen in a cut and polished sample. The apparent universality of the 595 nm line in treated diamonds, and its absence in untreated diamonds has therefore been used has a guide to a diamond's history, and indeed the Gemological Institute of America has based its certificates of authenticity on this criterion¹. This note draws attention to the fact that absorption at 595 nm in a treated diamond may be destroyed without significantly altering the colour of the stone.