Heat Treating the Sapphires of Rock Creek, Montana

Abstract

Renewed interest in large-scale commercial mining of Montana sapphires motivated a detailed study of the heat treatment of sapphire from the Rock Creek deposit. During the course of this systematic study, over 75,000 stones were processed. It was found that approximately 65%-70% of the pale blue, pale green, and near-colorless sapphire could be converted to well-saturated blue and yellow colors. Some improvement in clarity was noted as well. Yellow hues were easily developed by heating in oxidizing atmospheres and were relatively insensitive t o time and temperature. Blue hues were developed by processing in reducing atmospheres. The relatively high iron content of Rock Creelz material restricts the range of reducing conditions that can be used to achieve optimal coloration because of hercynite precipitation in the sapphire,

Gemstone Enhancement and Detection in the 1990s

“…For those laboratories that provide locality-of-origin determinations, such modifications-coupled with the greater number of corundum sources found during the decade-only added to the complexity of determining the geographic origin of a ruby or sapphire (see, e.g., Schwarz et al, 1996). However, a number of articles did address the techniques used and the effects of heat treatment on sapphires from localities such as Kashmir (Schwieger, 1990), Sri Lanka (Ediriweera and Perera, 1991;Pemadasa and Danapala, 1994), Montana (Emmett and Douthit, 1993), Australia (Themelis, 1995), and Mogok (Kyi et al, 1999).…”

Beryllium Diffusion of Ruby and Sapphire

“…A good general review is provided by Rossman (1987, 1988);Häger (2001) orangy yellow (Kvapil et al, 1973;Schmetzer, 1981;Schmetzer et al, 1983;Boiko et al, 1987;Emmett and Douthit, 1993;Häger, 1993 …”

“…The striking color change in the geuda material was caused by the dissolution of rutile inclusions in the stone, and by the inward diffusion of hydrogen from the reducing atmosphere. The importance of hydrogen diffusion was not recognized until much later (Emmett and Douthit, 1993). Eventually, tons of previously worthless geuda corundum were converted to marketable transparent blue sapphire.…”

“…The benefits of this process with geuda sapphire led to successful experimentation with many types and colors of sapphire (e.g., Crowningshield and Nassau, 1981;Keller, 1982;Themelis, 1992;Emmett and Douthit, 1993) as well as off-color ruby. As a result, the vast majority of rubies and sapphires traded today have been heat treated.…”

"Diffusion Ruby" Proves to Be Synthetic Ruby Overgrowth on Natural Corundum

“…It is particularly important that the natural starting material contain low relative concentrations of titanium and iron so the host will not develop a distinct blue coloration during the heating process, thus producing an overall purple coloration once the synthetic ruby overgrowth is added. The orange color concentration in the interior of one sample is attributed to trapped-hole color centers associated with naturally occurring magnesium (Mg 2+ ) that were created during the heating/overgrowth process (see, e.g., Emmett and Douthit, 1993).…”