Chrysoberyl

 Chrysoberyl

Chrysoberyl is beryllium aluminium oxide BeAl₂O₄ and forms highly characteristic transparent to translucent pseudo-hexagonal, multiple con- tact and penetration twinned or prominently striated short prismatic crystals in the orthorhombic system. The hardness is 8.5 and there is a distinct cleavage in one direction. While the lustre is vitreous crystals may be opaque and chatoyant with oriented inclusions. Alexandrite is most commonly dark green with a colour change to raspberry red when viewed successively in daylight or its artificial near-equivalent and incandescent lighting. Chatoyant chrysoberyl shows an eye when translucent and cut with a dome (cabochon) which allows incident light to reflect with a sharp eye from parallel included crystals or channels to the upper surface of the finished stone. Transparent chrysoberyl apart from alexandrite is often very bright and pleasing golden to greenish yellow, sometimes dark orange or brown, rarely colourless.

The RI for the alpha, beta and gamma rays is 1.746, 1.748 and 1.756

respectively, biaxial positive with birefringence 0.008–0.010. The SG is in the range 3.71–3.72.

Pleochroism is strong, and in alexandrite, shows red, yellow-orange and emerald-green colours. The absorption spectrum of yellow-green or dark brown chrysoberyl is notable for the broad absorption band centred at 444 nm in the violet. In dark specimens the observer may also see bands at 505 and 485 nm and in very brown stones the general absorp- tion of the blue and violet conceals the 444 band. Before examining the absorption spectrum of alexandrite we should first remember to consult Anderson and Payne’s text (passim) on absorption spectra and then the stone’s strong pleochroism where the highest RI direction is green, the

 

lowest red or purple and the intermediate ray orange. The observer will need to note that the absorption spectrum shows some differences depending upon the direction examined. In the green ray the narrow doublet at 680.5 and 678.5 nm is particularly prominent with the 680.5 nm band the stronger. Weaker lines can be seen at 665, 655, 649 and 645 nm. The broad central absorption extends from about 640 to 555 nm and appears to show two deeper areas of concentration. Blue and violet are completely absorbed from about 470 nm.

In the red or purple ray the doublet is weaker and the 678.5 nm band is now the stronger. Two other lines, at 655 and 645 nm are seen in the red. The broad absorption is now 605–540 nm, well away from the red and accounting for the change to red with appropriate lighting. A clear line can be seen at 472 nm, sometimes accompanied by a weaker line at 468 nm. The absorption of the blue is now seen at 460 nm. Anderson and Payne (1998) carried out the investigations so far quoted on Siberian alexandrite in which the colour change is stronger (more chromium) than that shown by Sri Lankan alexandrite. In Sri Lankan alexandrite the broad absorp- tion zones are less intense than in Siberian (and the recently discovered Brazilian) alexandrite.

Between crossed filters most alexandrite will show a distinct red (more than might be expected in Sri Lankan specimens) and any such response from a chrysoberyl entitles it to the name alexandrite, providing there is some colour change. Compared with ruby the emission doublet in the red is far less strong but can be seen with careful observation and appropriate lighting. The strong pleochroic colours red, yellow-orange and green are not associated with the colour change.

 

Cat’s-Eye

Cat’s-eye chrysoberyl is the finest of its kind; while a number of other species may show eyes when appropriately cut, their eyes are far coarser and the flat back of many chatoyant specimens enables an RI test to be made. Chrysoberyl can be tested with the spectroscope. The name ‘cat’s- eye’ is used on its own for chrysoberyl only, all other chatoyant stones needing the species to be used adjectivally. Occasionally a chrysoberyl will show a four-rayed star. The finest cat’s-eyes should show a very sharp blue-white eye against a dark honey-coloured background.

While chrysoberyl is a characteristic mineral of granite pegmatites associated with mica schists or reaction zones in ultramafic rocks, gem crystals are found most often in placers. Brazil, long a producer of the best and brightest yellow-green chrysoberyl, has in recent years become the producer of magnificent alexandrite from the Lavra de Hematita

Cat’s-Eye

 

field in Minas Gerais. The best material from this deposit equals or even surpasses alexandrite from the Takovaya emerald mines, north-east of Yekaterinburg [Sverdlovsk], Russia. The colour change is pronounced in the best alexandrite from both sources. Alexandrite was named in 1982 for Tsar Alexander II. Crystals are most commonly found as star-shaped twins (trillings) reaching 2 cm across in some examples. The host rock is a mica schist.

Fine yellow chrysoberyl is found in the Sanarka district of the southern Urals though the best examples come from Jacuda, Brazil. Interesting small, included alexandrites but with a marked colour change have been mined in Zimbabwe in the Smabula Forest area and from Lake Manyara, Tanzania. A colourless chrysoberyl has been found in Myanmar and a faceted example forms part of the ACD Pain collection in the Natural History Museum, London. A brown specimen from a pegmatite in the Harts Range of central Australia gave an RI of 1.765, 1.772 and 1.777 and showed an intense absorption band in the violet centred near 447 nm with another at 508 nm. Chrysoberyl has also been found at Anakie, Queensland.

The finest cat’s-eyes are found in Sri Lanka though many commercial specimens are exported from Brazilian sources.

 

Inclusions

Minute rutile crystals forming groups running parallel to one of the crystal axes are characteristic of many chrysoberyls from India and may occur in material from elsewhere. Liquid-filled cavities with gas bubbles are also characteristic. Stepped planes, not always easy to see, are characteristic of some yellow crystals.

 

Synthesis and Simulation

Alexandrite has been synthesized by a number of different processes, including Czochralski, floating-zone and flux. Inamori in Japan has also produced an alexandrite cat’s-eye. Pulled crystals sometimes advertized as alexandrite in rockhound-type journals are traps for the unwary amateur as they contain insufficient chromium to produce an alexandrite colour.

Identifying features of these synthetic chrysoberyls include undigested flux from crystals grown in precious metal crucibles, some gas bubbles, wavy fibres, weak yellow fluorescence from the crystal surface and sup- plemented by a red-orange response below the surface; this effect is seen under SWUV. The reader may consult O’Donoghue and Joyner (2003 passim) for further information.

 

Crystals grown in Russia may show flux-filled or partly filled nega- tive crystals, some showing surface crazing and appearing white or pale yellow. Some crystals have shown ‘Venetian blind’ or shuttering effect. No natural chrysoberyl shows any such inclusions.

 

Imitations

Alexandrite is well worth imitating and prospective purchasers should bear in mind that even the finest natural specimens do not show a very marked colour change; the green is dark and the ‘candle-light’ so often recommended   for   viewing   alexandrite   might   make   the   stone   as a whole hard to see at all. The flux-grown synthetics show too strong a colour change but the colours are fairly accurate. This cannot be said of the flame-fusion-grown vanadium-doped alexandrite simulant which looks far more like amethyst. Even today judicial proceedings feature this well-known (and quite attractive) product and there is always some expert witness to demonstrate how far gemmological education still has to go. Sometimes the specimen in question is not even forthcoming. A dark green Verneuil-grown spinel said to be made to imitate green tourmaline did show a dull red colour change and this could be passed off as alexandrite. Neither birefringence nor dichroism would be possible.

Composites with a dyed central gelatine filter have also been used as

alexandrite simulants.

While a number of natural yellow stones may be chatoyant enough to provide a reasonable cat’s-eye imitation, in no examples is the eye really sharp. In the ingenious ‘Cathay stone’ the eye is sharp though its host-glass is soft. Nonetheless this is an attractive and convincing product, at least through the shop window! Two types of glass are used, one in the form of hexagonal fibre-optic bundles. ‘Victoria cats-eye’, another glass product, is subject to devitrification and may on occasion show chatoyancy but it would be vague at the best. Transparent green and yellow chrysoberyl is neither imitated nor synthesized.