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| We recently had an opportunity to test a gbicolourh faceted stone shown in the photo. Our investigation revealed that the stone is a Maxixe-type beryl. The characteristic feature of the stone is introduced. A deep blue beryl was found in the Maxixe mine in Minas Gerais state, Brazil, in 1917. The stone is thought to be naturally coloured by natural irradiation, but the colour was unstable to light and heat, and it was called Maxixe beryl. Similar beryl appeared on the market around 1973, but the material was in fact artificially irradiated and thus called Maxixe-type. @ Although the cause of the colour is the colour centre produced by irradiation in the both case, defects of the colour centre are presumably related to NO3 in original Maxixe beryl, and to CO3- in Maxixe-type beryl. The colour, in both materials, is unstable and its possible colour fading is controversial (see the April 1996 issue of GEMMOLOGY for detail of Maxixe beryl and the September 1996 issue for the colour fading tests). There is little chance to come across stones that have characteristics of original Maxixe beryl in the gemstone market these days, although deep blue or green-yellow faceted stones of Maxixe-type (artificially irradiated) are often sent to our laboratory for identification. The stone in the title we introduce this month was, according to the client Mr. Hiromichi YAMAMOTO of Jewel Climb, purchased in Bangkok as a bicolour beryl from Brazil. |
Its refractive indices were 1.579`1.586 with the birefringence 0.007 and its SG was 2.70, all of which were slightly higher than those of general aquamarine or yellow beryl. Pleochroism was distinct and deep blue colour was observed along the optic axis direction in blue section of the stone, which is a unique characteristic to Maxixe and Maxixe-type beryl with ordinary ray showing the deeper colour. General aquamarine shows deeper colour in extraordinary ray. With a handy-type spectroscope, several absorption bands were recognised from red to yellow part of the spectra in both green and blue sections of the bicolour stone. Those absorption bands were proved to be on 689, 644, 625, 605, 588 and 573nm by measurement on a spectrophotometer, which corresponded with the absorption characteristics of Maxixe-type stones. The stone was inert under long wave ultraviolet light, but chalky yellow-green fluorescence was observed partly in blue section under short wave ultraviolet light. We made a comparison of elements other than major ones of beryl in blue and green section by compositional analysis with X-ray fluorescence, and the test showed high content of Fe and Zn in green section and Rb and Cs in blue section. Under magnification inspection, liquid inclusions and growth lines parallel to the optic axis (probably along one of prisms) were recognised, and the boundary between blue and green sections were harmonious with those growth lines. |
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