342 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



given under hessonite ; it may be stated here, however, that the distinction is based on the' 

 fact that hessonite is singly refracting and hyacinth doubly refracting. 



The very pale straw-yellow or colourless zircons from Ceylon are also cut as gems, and 

 are called by jewellers "cei'konier" or jargon (jargoon). Though this variety of zircon 

 occurs but rarely in nature, it can be produced artificially to any amount by heating 

 hyacinth of the ordinary colour, this being very easily decolorised by heat. Thus, a stone 

 on being brought near to the tip of a blowpipe flame suddenly loses its red colour, and 

 becomes colourless or tinged with grey, sometimes with rose-i'ed or straw-yellow. The 

 same change of colour takes place when hyacinth is heated in a glass-tube. If the experiment 

 is made in a dark room it will be observed that the stone, although at a temperature below 

 red-heat, suddenly emits a phosphorescent liglit and becomes decolorised. Other important 

 changes take place at the same time ; the specific gravity of the stone is increased by O'l, or 

 even moi'e, while the lustre, always brilliant, becomes still more so. So brilliant indeed are 

 these ignited, colourless or pale-coloured hyacinths, that when cut as rosettes it is impossible 

 for any one other than an expert to distinguish them on mere inspection from diamonds, and 

 there is no doubt that they are occasionally passed off as such. The stones found at Matura, 

 in Ceylon, in the eighteenth century, were, in fact, regarded as diamonds of inferior quality, 

 and were known as " Matura diamonds." The diamond, like hessonite, differs from hyacinth 

 in that it is singly refracting ; its hardness, moreover, is, of course, enormously greater, and 

 its specific gravity is less ; in the heaviest liquid (sp. gr. = 3-6) diamond floats and hyacinth 

 rapidly sinks. 



The phenomena attending the decolorisation of hyacinth have been somewhat closely 

 investigated with a view to learning the nature of the colouring substance. It is found that 

 crystals when heated in the presence of oxygen — for example, in the oxidising flame — do not 

 lose their colour completely, but become paler. When, on the other hand, they are heated 

 out of contact with oxygen — for example, in the reducing flame — they are completely 

 decolorised ; and if these decolorised stones are again heated in the presence of oxygen they 

 assume a pale red colour. Moreover, it has been observed that a very strongly ignited 

 hyacinth becomes dark brown in colour. From these observations it is inferred that the 

 colour of zircon is due to the presence of iron ; as a matter of fact, every specimen of zircon 

 hitherto analysed has been found to contain small amounts, ranging up to 2 per cent., of 

 iron oxide. 



The colour and lustre of some hyacinths is liable to change even at ordinary temperatures 

 if the stones are exposed to light, especially to the direct rays of the sun. In some cases 

 the colour becomes pale, while in a few stones it changes to a brownish-red which gradually 

 becomes more decidedly brown. At the same time the adamantine lustre becomes gradually 

 more vitreous in character. Such altered stones, if kept in darkness, will recover their 

 original colour and lustre to a large extent, but not altogether. Although these changes 

 are not undergone by all hyacinths, it is advisable not to expose the gem to sunlight 

 unnecessarily. 



Zircon is more strongly refracting than any other precious stone with the exception of 

 diamond. Being a tetragonal mineral it is doubly refracting ; the values for the greatest 

 and least refractive indices of a crystal of a hyacinth from Ceylon were determined to be 

 1-97 and 1-92 respectively. The difference between the two, 0-05, is a measure of the 

 double refraction, which is thus specially strong. On the other hand, the refractive indices, 

 for different colours do not differ much, hence the dispersion of zircon is small and its play 

 of prismatic colours correspondingly insignificant, so that, although in brilliancy and lustre 

 t may compare with the diamond, yet in the former respect no comparison is possible. 



