Dehydration of Diaspore and Goethite during Low-Temperature Heating as Criterion to Separate Unheated from Heated Rubies and Sapphires

Abstract

Gem-quality rubies and sapphires are often commercially heat treated at about 800 °C or higher to enhance their color and clarity, and hence quality. For this study, selected corundum samples containing diaspore and goethite inclusions were heated step-by-step to a maximum of 1000 °C with the aim of monitoring the dehydration and phase transformation of these oxyhydroxides to corundum and hematite during heating. Based on our experiments and in agreement with the literature, the dehydration of diaspore in corundum occurs between 525 and 550 °C, whereas goethite transforms to hematite between 300 and 325 °C. As both diaspore and goethite may be present as inclusions in rubies, sapphires, and other corundum varieties (e.g., pink sapphires, padparadscha), these dehydration reactions and phase transformations can be considered important criteria to separate unheated from heated stones, specifically in cases in which other methods (e.g., microscopy, FTIR) are unsuccessful.

Editorial for the Special Issue “Gem Characterisation”

“…Treatments to improve the colour, appearance and/or durability of gems have been used for several years; over recent decades, these have increased and become more sophisticated [10][11][12][13]. In the present issue, four studies on the treatment of gems are published [14][15][16][17]. Zhou et al [14] present the results of the method they developed for the hightemperature copper diffusion process for the surface recolouring of faceted labradorites.…”

“…Detection of this treatment is not easy as the temperature is relatively low and induces little change in internal features, but a combination of UV-Vis-NIR spectroscopy and trace element chemistry provided by LA-ICP-MS might give evidence of such treatment. Two papers on the detection of the low-temperature heat treatment (i.e., below 1200 • C) of corundum by studying inclusions are also published in this Special Issue [16,17]. Krzemnicki et al [16] present a study on rubies and sapphires containing diaspore and goethite inclusions.…”

“…Two papers on the detection of the low-temperature heat treatment (i.e., below 1200 • C) of corundum by studying inclusions are also published in this Special Issue [16,17]. Krzemnicki et al [16] present a study on rubies and sapphires containing diaspore and goethite inclusions. Based on their experiments and in agreement with the literature, the dehydration of diaspore in corundum occurs between 525 and 550 • C, whereas goethite transforms to hematite between 300 and 325 • C. As both diaspore and goethite might be present as inclusions in gem-quality corundum, these dehydration reactions and phase transformations can be considered important criteria to separate unheated from heated stones.…”