◆Enhancement on ruby by heat   Heating process on gemstones aiming at colour improvement has a long history, and many gemstones nowadays are heated on a routine bases. Ruby is a typical example of them. Ruby is heated in higher temperature than other gemstones and may continue to be re-heated until the colour improves to a required level. Repetition of heating in high temperature causes heat stress to the structure of corundum, and also changes patterns of distribution of crystal defects or impurities.

Universal inclusion in natural ruby is rutile caused by exsolution. When they are observed as narrow needles under magnification, the stone has at least not been undergone through the heat as high as the fusing point of rutile. However, rutile is scarcely observed in the form of needles in ruby and they usually appear as dots or clouds even in unheated stones. This cloud inclusion shows indistinct boundaries between distributed and not-distributed sector of minute scatterers on laser tomography in an unheated stone (photo-4 and 5). Contrary to this, in a heated stone the minute scatterers gather in certain areas to show sharper sector boundaries (photo-6 and 7).

Photo-4(left) and 5(right): Minute scatterers in unheated ruby. Photo-6(left) and 7(right): Minute scatterers in heated ruby showing distinctive sector boundaries.

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Some heated ruby may partly show patchy fluorescence of orange colour against red fluorescence of the whole stone (photo-8). Such examples have never been observed in unheated stones so far. 　 Laser tomography on some heated stones sometimes show linear defects (=dislocation) which can not be seen under standard gemmological microscopes. These defects are observed as a bunch of lines starting from a certain plane inside a crystal, and most of them reach to the outer rim of a faceted stone (photo-9, 10 and 11). Dislocation is known to be caused during growth process of a crystal and to be moved its position by foreign stress that is applied after the growth. From this fact, it can be gathered that originally existing inhomogeneous part of a crystal accepts acquired stress most probably by heating and the dislocation move towards outside and proliferate to release the stress.	Photo-8: Orange patches of fluorescence seen in some heated ruby.
Photo-9(left),10(centre) and 11(right): Dislocation seen in heated ruby.
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