2007: A real treat from a gemological genius. Good tips for students of gemology, lab gemologists, gem dealers, jewelers and those who love colored stones.
(via The Journal of Gemmology, Vol.XIV, No.3, July 1974) B W Anderson writes:
(being the substance of a talk given to the Gemmological Association of Great Britain at Goldsmith’s Hall on 29th October, 1973)
In the talk I gave in January I described our early struggles in the Precious Stone Laboratory from 1925 onwards, first in learning our main job of pearl testing and later in improving and extending the techniques for testing gemstones of all kinds. Tonight, in continuing the inside story of the Laboratory I am proposing to stick pretty closely to one main theme rather than risk getting lost in recalling a host of little incidents: the theme being the story of discoveries of new gem varieties and new gem minerals in which we were lucky enough to be involved to a major or minor extent.
At present time there are some 2500 separate mineral species known to science. Each year a number of new names are added, but most of them are not only very rare but quite insignificant in form. One sometimes feels rather sorry for some worthy scientist whose name is given by its discoverer as compliment to some very indifferent mineral! The small importance of most of these in indicated by the fact that in a standard textbook such as the 1971 edition of Dana’s Manual of Mineralogy only some 200 species were considered worthy of description.
But the discovery of a new gem mineral is a rare event, for it implies that the specimens found are at least large enough to be cut as stones suitable for jewelry, and usually that they are transparent and pleasingly colored. From the trade point of view the recovery of new varieties of an already known mineral may be much more important. One has only to think of demantoid (1878), kunzite (1902), and tanzanite (1967) as instances of this.
Gahnospinel
Our first investigation into stones which had not previously been described concerned certain blue spinels from Ceylon which had a normal appearance but which were found to have a refractive index, and particularly a density, which was far higher than any quoted in the literature. C J Payne and I had already noted several such anomalous stones, but the real challenge came in 1935 when T W Oliver, who was then a gemology student at Chelsea Polytechnic, showed me a blue spinel which puzzled him in having a refractive index of over 1.74 instead of customary 1.715 or 1.72 of a spinel with so pale a lavender blue. In the laboratory we found the actual figures to be 1.7432 for the refractive index (using the minimum deviation method), and the density to be 3.947, which was even more startling.
The hunt was now on: we set to work in earnest to search for comparable stones, working through parcels of Ceylon stones borrowed from the rich stock of E Hahn & Sons, who were in those happy days established in 26, Hatton Garden. We also segregated by means of Clerici solution high density blue spinels from samples of the Ceylon gem gravels. The rarity of these anomalous stones is indicated by the fact that of over 300 spinels examined, only four had densities above 3.85.
Eventually we had in our hands a graduated range of blue spinels ranging from No.1 specimen, which was a pebble polished as a prism by Mathews Lapidaries, which gave us the measured figures of 1.7469 for refractive index and 3.981 for density, down to No.22, which had the normal values of 1.7153 and 3.584 respectively.
We realized that the replacing element causing these enhanced figures had to be one known to form a ‘spinel’ on its own and one which would have no influence on the color. Our guess that this element was zinc soon proved to be correct. We prepared a graph on which we plotted the density and refractive index of pure magnesium spinel and the corresponding figures (4.625 and 1.805) for a man-made zinc spinel, known in nature as the mineral gahnite. The zinc-rich spinels of our newly discovered series found to fit satisfactorily along the line between the two points and were well away from the line leading from the plot for magnesium spinel to that for the iron spinel, hercynite. Our ‘gahnospinels’, as we christened them, varied in color from pale to dark blue, according to their content of ferrous iron, but this had very little influence on their properties. Any considerable influx of iron causes spinel to become black and opaque and fit only for mourning jewelry. Ceylonite and pleonaste are variety names which have been used for such stones, typical values for which are 3.8 for density and 1.78 for refractive index.
We also used a small grating spectrograph made for us by Bellingham and Stanley to record the emission spectrum of small samples of stones selected from our series, fusing them in a purified carbon arc for the purpose. The spectra not only showed the expected increase in the strength of the zinc emission lines in the higher density samples, but also revealed the unexpected fact that all blue spinels from Ceylon contain at least a trace of zinc.
Dr Max Hey, the highly skilled analyst in the Mineral Department of the Natural History Museum, kindly carried out a quantitative analysis of one our ‘top’ stones and found it to contain 18.21% zinc oxide, 16.78% magnesium oxide, and 1.93% ferrous oxide—to which last the color and absorption spectrum were due. We then had enough data to justify a paper on these stones, which was published in the Mineralogical Magazine—this being the Journal of the Mineralogical Society, which is the accepted vehicle for contributions to mineralogy in this country.
This whole investigation was ideal for our first serious incursion into mineralogy. In those far-off days specimens for our purpose were readily and cheaply obtainable (Ceylon, it may be remembered, was still under the British rule); we had recently acquired a Beck table spectrometer, which enabled us, with suitably cut stones, to measure refractive indices and dispersions to four decimal places, and we were able to make accurate density determinations even on small specimens by suspension in Clerici solution followed by measurement of the R.I of the solution to our places of decimals in a hollow prism and working from a graph we had prepared showing the connexion of the density and R.I of this solution. It also gave us practice in an essential part of all research work—the art of ‘consulting the literature’ to ensure that our findings had not been already written by other workers.
A brief word on this last process may be of help to beginners in this fascinating business called research. Looking round the shelves laden with scientific journals in a big science library, such as the one in Southampton Buildings off Chancery Lane, which was formerly the Patent Office Library and is now the Science Library of the British Museum (proximity to which was not the least of our blessings), one might despair of making a thorough search. But it is not so difficult as it seems. For the past few decades at least, Mineral Abstracts have existed and a rapid search through the indexes of their more recent volumes under ‘spinel’, say, will lead you to papers on the subject that interests you. Consulting the latest of these will provide you with all the necessary references up to that time: the author will have done that work for you. A knowledge of German may be helpful, but copying facilities are provided by the library, and in ten minutes you can be provided with a photocopy which you can brood over at your leisure.
Before leaving the subject of gahnospinel I might mention that the highest figures yet encountered were in blue spinel sent for a routine test in 1964. This had density 4.06 and refractive index 1.7542. It is hardly likely that even so extreme a case might be confused with sapphire, but it is not uncommon for stones containing only a small proportion of zinc to have refractive indices around the 1.728 mark—a value associated in the mind with synthetic spinel.
The Pleasure Of Discovery (continued)
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