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HEAT TREATING RUBY AND SAPPHIRE: TECHNICAL ASPECTS By Kurt Nassau Ruby and sapphire of natural or synthetic n recent months, a variety of heat-treated sapphires origin can be heat treated to improve I and rubies with gemological characteristics different asterism; to remove asterism or silk; to from any previously seen have appeared on the market. improve, add, or remove color; and even Contradictory rumors abound regarding the different to alter imperfections. At least nine manners and methods of treatment. distinct processes can be identified, Examples of heat treatment methods that have been although several may occur simultaneously. Some of these treatment used to improve the appearance of gemstones include the methods correspond to natural processes conversion of green aquamarine to blue; the "pinking" of and may not leave any evidence of their brown chromium-containing topaz; the "crackling" of use; others do not correspond to natural quartz; the reddening of yellow agate, carnelian, and processes and are readily identifiable. tiger's-eye; the development of the blue color of tanzan- ite; and the conversion of amethyst to citrine or to "greened amethyst." Such processes may be termed heat- ing, burning, annealing, firing, high-temperature soaking, and so on. The use of these types of heat treatment is usually not specified. The reaction of synthetic corundum (Nassau, 1980a) to various types of heat treatment has been studied widely, and the results are equally applicable to the natural ma- terial. Although most of the treatment methods described here have been known for some time, their use to im- prove rubies and sapphires has become widespread only recently. The important parameters in any type of heat treat- ment are: 1. The temperature-time relationship ABOUT THE AUTHOR 2. The oxidation-reduction conditions Dr. Nassau is a research scientist residing in 3. The presence of chemical substances that can in- Bernardsville, NJ, He does not wish to be contacted on any aspect of this article. teract with the gemstone Acknowledgments: The author is grateful to Heat treatment of corundum can affect the presence of Robert Crowningshield, of the GIA Gem Trade milkiness and asterism, the color, and even the internal Laboratory in New York, for helpful discussions; structure (inclusions as well) of the material. A number and to Robert E. Kane, of the GIA Gem Trade Laboratory in Los Angeles, for gathering the of separate processes have been distinguished, although illustrations and writing the figure legends, several may be performed simultaneously. The exact %'I981Gemological Institute of America temperature, duration of treatment, and chemicals used Heat Treating Ruby and Sapphire GEMS & GEMOLOGY Fall 1981 121 for each process will depend on the specific ma- PROCESS 1: terial being heated; considerable variation must DEVELOPMENT OF be expected with corundum from different lo- POTENTIAL ASTERISM calities. Some natural sapphire and ruby, as well as some Nine possible treatment modes are summa- synthetic material intended for star use, contains rized in table 1 and examined in turn below. Iden- a significant amount of titanium oxide. If such tifying characteristics of such treatment methods corundum cools fairly rapidly from its growth have been discussed in detail by Crowningshield conditions, the material remains clear; the tita- and Nassau (1981);therefore, they will be touched nium oxide is in "solid solution" in the form of on here only briefly. Tifi (titanium sesquioxide) replacing some of It should be noted that no consensus has yet the A120y(aluminum oxide). been reached as to whether any of these types of If such material, which typically contains only treatment need to be disclosed to the buyer. The a few tenths of a percent titanium oxide, is held parallel is sometimes drawn that since heat treat- at between llOO° and 1500° for some time ment is not customarily disclosed with other (say, 1300° for 24 hours), particularly under gemstones, such as heated blue aquamarine, why mildly oxidizing conditions, the TizOg converts should such disclosure be necessary with sap- to TiOz (titanium dioxide) as follows: phires and ruby? There may be validity in viewing some types of treatment in this way, especially those that mimic natural processes (as is the case with aquamarine). However, other treatment In most cases, the TiOz will then form needles of methods-diffusion in particular-do not have rutile within the corundum and thus produce as- any parallel in nature. The results of diffusion terism. This process, which was patented for Linde treatment, for example, are readily recognized, Air Products Company by Burdiclz and Glenn and remind one more of Lechleitner synthetic (1949))is used to create all synthetic stars in co- emerald overgrowth on a natural beryl than of rundum (Nassau, 1980a), and the analogous pro- heated aquamarine. cess also occurs in nature. In fact, if a piece of TABLE 1. Heat treatment processes used on sapphires and rubies Treatment Specific group processa Result Heating only 1. Moderate temperature Develops potential asterism (130OoC) 2. High temperature Removes silk and asterism (160OoC), rapid cooling 3. Reductive heating Develops potential blue color (16OO0C) 4. Oxidative heating Diminishes blue color (16OO0C) 5. Extended heating Diminishes Verneuil banding (1800°C and strain Heating under unknown 6. ? Introduces fingerprint conditions inclusionsb Diffusion of impurities into 7. Add TiO, Produces asterismb the material (extended 8. Add Ti02 and/or Fe203 Produces blue colorb heating at 1800°C 9. Add Cr203,NiO, etc. Produces other colorsb ^Treatments 1 through 4 correspond to processes that also occur in nature; treatments 5 and 6 are used on synthetic material; treatments 7 through 9 do not correspond to natural processes. The temperatures given are representative only and will depend on the nature of the material and the length of time they are used. 'Effect is limited to a region near the surface. 122 Heat Treating Ruby and Sapphire GEMS & GEMOLOGY Fall 1981 Figure 1. Left, milky white "geuda" corundum from Sri Lanka; photograph by Tino Hammid. Right, intense blue color induced in Sri Lanka 'geuda" by the heating method described in process 3; photograph by Michael Havstad. natural corundum contains sufficient titanium the rutile will dissolve in the corundum by the oxide but was not heated sufficiently in nature to reverse of process 1: develop a good star, perhaps showing only fine "sillz," asterism can be developed by means of such heat treatment. Thus far, however, this pro- cess appears to have been used on natural corun- When all the silk has disappeared, the corundum dum on an experimental basis only. is cooled fairly rapidly, perhaps 30Âper minute If rutile needles in natural corundum are too (Fallzenberg, 1978), so that rutile needles do not coarse to provide a good star, process 2 can be re-form as in process 1, Asterism may be removed used to return the titanium oxide to solid solu- in a similar manner. tion by the reverse of equation (1))and process 1 The suggestion by Sasalzi (1980)that this form can be used subsequently to form star-causing of heat treatment might be conducted success- needles as described above. The heating condi- fully at 1000°(538OC) implies a temperature that tions of process 1 are relatively mild and often is as unreasonably low as the 4000° reported by leave no evidence by which this treatment may Tombs (1980)is unreasonably high (it is above the be recognized with certainty. melting point!). Tombs also proposes that the silk It should be noted that the AlaOg-TiOa phase may originate from substances other than tita- diagrams of Lang et al. (1952) and Lejus et al. nium oxide, for example, from a-corundum pre- (1966) show an intermediate compound Al2TiO5, sent in fi-corundum. But ordinary corundum is and that this and other compounds have been sug- a-corundum. Such unsupported suggestions must gested as representing the needles (e.g., Phillips be discounted unless and until concrete evidence et al., 1976). However, detailed examinations becomes available. Nevertheless, it is possible always have pointed to the rutile form of tita- that heat treatment may cause other inclusions nium oxide as the principal component of the or defects to disappear by a process of solid so- needles (e.g., Nassau, 1968; Phillips et al., 1980). lution similar to that involving titanium oxide. Neither of the phase diagrams cited considers the Oughton (197 1) quotes another unusual sugges- solid solubility of TiOz or Ti203in A1203,which tion, namely that liquid may be used to fill the must, on theoretical grounds, be present (see also, hollow tubes that cause sillz, with the effect wear- for example, Bratton, 1971). ing off after about 12 months. This might work if only there were hollow tubes! PROCESS 2: Process 2 appears to be used widely on silky REMOVAL OF SILK OR ASTERISM Australian sapphire and on milky white to pale If corundum containing silk or asterism caused by blue "geuda" corundum from Sri Lanka, which is rutile needles is heated to a sufficiently high tem- turned blue by the simultaneous use of process 3 perature, typically between 1500° and 1700°C (see figure 1).Gunaratne (1981) refers to this pro- Heat Treating Ruby and Sapphire GEMS & GEMOLOGY Fall 1981 123 Magnified 27 X. Photomicro- graph by Robert E. Kane. cess, but most of his discussion of the genetic not been properly repolished (Crowningshield and history of geuda material and of the treatment Nassau, 1981).However, none of these character- used must be taken as purely speculative, since istics may be present in a given treated stone, or it is in great part inconsistent with known data. some may be present in stones that have not been Cloudy Burma rubies, too, are said to be improved treated.

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