2007: The amazing thing is despite the limitations, the trade still insists for origin certification and a few gem testing laboratories are more than happy to provide their services at a cost.
(via Gemological Digest, No.3, No.1, 1990) Edwin Roedder writes:
Richard W Hughes has asked for my comments on the question of the use of gemstone origin reports and in particular on his paper on that subject in this same issue. As a mineralogist-geochemist who has only seen the fringes of the field of gemology over the years, it is not appropriate for me to discuss the uses being made of origin reports in the gem trade. I can only say that Mr Hughes paper seems to present a very coherent, well-documented and cogent review of the subject. Although I have read many scientific papers dealing with the inclusions in gems and the possibility of their use in determining origin, I have never seen an actual origin report, and hence my only comments deal the validity of the inclusions criteria that might be used in issuing any such report. These comments are quite apart from the difficulties mentioned by Mr Hughes of obtaining material of verifiable origin on which to establish such criteria.
I will discuss two aspects: first, the effects of the inherent variation in the inclusion population in samples from a given locality, and second, the general similarity of the inclusion populations in samples from many different localities.
Variation of the inclusion parameters within samples from a given locality
Many different kinds of inclusions, both fluid and solid, can be trapped in a given mineral from a given locality. The reasons for this wide range lie in the basic nature of the origin of inclusions. Primary inclusions formed during the growth of the host crystal, can trap any solid, liquid, or vapor phases that happen to be present at the time of growth. The much more common secondary inclusions are trapped as a result of healing of fractures in the host crystal that form at some later time; this later time may be immediately after completion of the growth of the host crystal, or as much as billions of years later.
Although I have studied a number of gem minerals, the bulk of my inclusion work has been on non-gem materials, but I am certain that the principals involved in the two types of materials are identical; the only real difference is in the rarity of gem minerals, and in the necessarily low abundance of inclusions in high quality stones. When I study the inclusions in many samples of a given mineral from a given locality (personally collected so there is no ambiguity as to sample origin), I usually find a large range in inclusion types, habits, and compositions in the different samples. The primary solid inclusions represent the trapping of those other solid phases that just happened to be present at the time of growth and which were enclosed rather than pushed aside by the growing crystal. If different solid phases were present in different part of the deposit, the solid inclusions that are trapped will differ similarly. Even the fluid that is trapped in primary inclusions can show gross differences in composition between the core and rim of a single host crystal, if the conditions have changed during that growth. Since some of the larger crystals in metamorphic rocks have recently been shown to have grown at rates in the range of only a millimeter in a million years, it would not be surprising to find that the composition of the fluids bathing the crystal had changed during its growth.
Secondary fluid inclusions (i.e most of the inclusions making up the feathers, etc in gems), represent one or more periods of fracturing and rehealing of the host crystal. It is very common to find that such fracturing has occurred at several different times, perhaps millions of years apart, and in the presence of very different fluids. So two different feathers in a single crystal may contain very different fluids.
Since such wide variations in the inclusions in different crystal or parts of a crystal from a single locality are so common, it may be difficult or even impossible to set up inclusion criteria that would permit a valid origin report to be written for a single sample of the host mineral from that locality. Various specific inclusion parameters seen in the single sample may match features seen commonly in other crystals from that same locality, but the wide range of possible parameter values, and the all-too-common atypical inclusions diminish the degree of confidence in any assignment of the sample to that locality. Although my work involves learning about the inclusions in samples from known localities, I would find it exceedingly difficult to have to work in reverse, as the gemologist must, and identify the locality of origin by the study of inclusions in a given sample. If presented with a single sample from one of the many localities I have studied, I might be able to venture an educated guess as to one (or more, as detailed below) possible localities, and in most cases I could say with some confidence that certain other localities were impossible, but I could not go beyond that. Perhaps in the future, when far more inclusion parameters can be determined with increased precision, this basic uncertainity might be reduced. Thus non-destructive chemical (and isotopic) analyses of the minor and trace elements in the fluids of inclusions might provide fingerprints that will greatly reduce the ambiguity in the assignment of origin, but will never eliminate.
Similarity of inclusion populations in samples from different localities
Gemstones, just as other natural crystals, have crystallized in a variety of geological environments, but even so, there is a relatively limited range for any given gem. Thus most gem quality corundums have formed in only a very few limited types of environment, such as certain slowly cooled igneous and metamorphic rocks. As a result, the overall range in composition of their inclusions, both solid and liquid is limited. A relatively rare phase as a solid inclusion may be a valuable but not necessarily unique indicator, e.g. the uranium pyrochlore referred to in Pailin sapphires. But many solid inclusions are of common minerals that might be expected in many localities, particularly in localities that are already similar geologically in virtue of the fact that a given gemstone is found in each. (On the other hand, certain solid inclusions can effectively exclude specific geologic environments. Thus fine fibrous amphibole inclusions are not expected in stones from a magmatic or high temperature metamorphic environment.)
Similarly, liquid inclusions in most minerals normally contain water + salt + CO2 major components. The amounts of the latter two may vary widely. For many years it was generally accepted that emeralds containing highly saline inclusions (i.e a saturated water solution plus a daughter crystal of NaCl) came from Colombian emeralds, but since then similar inclusions have been described in emeralds from other localities. CO2 is a major component in the fluid inclusions in many rock types that bear gem corundum and hence is not definitive of any specific locality.
If all the above caveats seem unduly pessimistic a to the significance of origin reports, please note that I have described only one of the many parameters that may be involved in judging the origin of a stone; I am not qualified to discuss color and its distribution and possible modification, fluorescence, density, refractive index, etc.
In closing, let me add one additional caveat. Origin reports must distinguish between synthetic and natural stones, as this is far greater importance in commerce than the difference between different localities, large as these are. In the past, the inclusions in gems have provided reliable criteria for differentiating. Synthetic stones generally had types of inclusions that were uncharacteristic of those from nature, and did not contain those that were characteristic of the natural stones. These differences are caused by the use of entirely different growth processes and conditions in the laboratory than were used in nature. Thus the manufacture of large corundum crystals in various colors by flame fusion provides high quality material for pennies per carat.
But it should be noted that the huge differences in value compared with natural stones provides a great incentive to eliminate these recognizable differences in the final product. Once careful study shows the nature of the inclusions present in (and believed to be characteristic of) the natural stones, it is highly likely that attempts will be made to grow synthetic stones by processes such that the inclusions will also appear natural. Presently available laboratory equipment can duplicate the pressure, temperature, and chemical environment under which any gem has formed in nature. This has been done with emeralds, which have been grown synthetically with strongly saline inclusions, and I have no reason to doubt that other natural appearing gems will be similarly produced and sold, if they have not been already.
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