The Pleasures Of Discovery

(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)

Sinhalite
Less than a year after the paper on taaffeite was read before the Mineralogical Society, Dr Claringbull was able to announce the establishment of yet another new gem mineral, which he named sinhalite from its origin in Ceylon. But on this occasion we were spectators rather than protagonists, though we were able to provide many specimens to aid the work, as sinhalite had been knocking around for sometime under the disguise of brown peridot. Robert Webster indeed nobly sacrificed part of his only specimen for Dr Hey to analyze. Whereas, when the taaffeite paper was read, all we had to show was two small cut specimens, Claringbull had a score of sinhalites to show (one giant of 75 carats) which had crept from their wrongly labeled packets for the occasion. There was even a small pebble of sinhalite which had been picked from a sample of illam by Dr E H Rutland.

Credit for the sinhalite discovery belongs properly to Dr George Switzer of the Smithsonian Institution, who took an X-ray powder photograph of scrapings from the girdle of a ‘brown peridot’ in the U.S Museum collection and found spacings which clearly differed from those of olivine. Knowing this, Dr W F Foshag (Switzer’s chief in the Institution) cast doubts on a specimen of ‘brown peridot’ in the Natural History Museum when Dr Claringbull was showing him round the mineral gallery, which gave rise to an energetic attack on the problem.

Sinhalite contains no silica, being a magnesium alumunium borate, MgAlBO4. Like peridot, it is orthorhombic, and its refractive indices, birefringence and density are very close to those of brown iron-rich peridots or olivines which are occasionally met with in Arizona and elsewhere; the chief difference being in the b index, which in peridot is nearly mid-way between the greatest and least indices, while sinhalite is clearly negative in sign. The absorption spectra are also very similar in the two minerals, but sinhalite shows an extra band at 4630 Angstrom. Sinhalites have been found in packets of golden zircons and of yellow chrysoberyls—they vary in color from pale straw yellow, but at their best are very attractive, being clean, transparent, and obtainable in important sizes. In fact, of all the newly discovered stones that I am talking about this evening sinhalite is the only one that has the slightest commercial importance. On the ‘anything you can do’ principle which I mentioned earlier, it was Burma which provided the first well-shaped sinhalite crystal, which C J Payne had the privilege of measuring.

Painite
In painite we have the rarest mineral of them all: in fact I find it rather amusing, considering that no cut stone exists (2007: today there are cut specimens available at affordable prices ), that a description of the stone occurs in at least five books on gemstones. The original dark red crystal, well-formed though rather waterworn, was found in one of the small ruby mines near Ohngaing village in the Mogok district of Burma. Mr A C D Pain, who suspected it might be something new, sent it to the laboratory for testing. The crystal at first sight looked as though it were tetragonal, but C J Payne, finding the prism angles to be exactly 60º realized that in fact it was hexagonal. It weighed 8.5 carats. The density was found to be 4.01 and the refractive indices 1.8159 for the ordinary and 1.7875 for the extraordinary ray, giving a birefringence of 0.0284. The hardness was measured as 8 on Moh’s scale by an indentation method. The dichroism showed a brownish red for the ordinary ray and deep ruby red for the extraordinary.

Permission was given for a thin slice to be removed from the base of the crystal for Claringbull and Hey to carry out the necessary X-ray and chemical work. Analysis showed the mineral to be borosilicate of calcium and aluminium, but it proved difficult to ascribe to it a definite formula. The specimen was justly named after its discoverer, and presented by him to the Museum where most of the work on it was done.

Chromium lines were visible in the red end of the spectrum, and the color was probably due to this, at least in part. In confirmation of this, the stone showed a red glow under crossed filters. It is difficult to judge how attractive a cut painite might be. In bulk, the color was too deep to be effective, but one might guess that small stones might look very much like Siam rubies.

The Pleasure Of Discovery (continued)