384. SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



river. The first find is said to have been made on Cooper's Creek. Other locaHties are the 

 northern part of Mount Tyre near Mount Marlow Station, Opal Range, Winton, Mayne 

 river, Canaway Range, Bulgroo, Mieavilla, and Listowel Downs. The mineral is thus widely 

 distributed, and, though inferior to Hungarian opal, is extensively used. 



FIRE-OPAL. 



Next in importance to precious opal comes fire-opal, so named from its fire-red colour. 

 It is likewise known as sun-opal, also on account of ibs colour; while the term girasol 

 is sometimes applied to this, as well as to other precious stones. 



In colour fire-opal ranges from an almost colourless or light brownish-yellow shade to 

 deep brownish-red. The finest tints are like those of some topazes or of hyacinth. The 

 stone is much paler in thin splinters than in thicker pieces ; also it is paler by transmitted 

 than by reflected light. Not infrequently several shades of colour are displayed by one and 

 the same specimen, the different tints passing gradually into each other or into perfectly 

 colourless opal. The colour depends, no doubt, upon the presence of a small amount of 

 iron oxide, the presence of which in fire-opal is shown by the analysis quoted above. 



The stone is translucent to almost perfectly transparent. The fracture is always 

 markedly conchoidal ; the lustre of a fractured surface is always high and can be enhanced 

 by polishing. When cut en cabochon or with facets, fire-opal, if not too light in colour, 

 gives a pi-etty gem. Fig. 10 of Plate XVI. represents a fire-opal in the rough condition and 

 Fig. 11 a cut stone. 



Many specimens of fire-opal exhibit a play of colours similar to that of precious 

 opal, when the two can be distinguished only by the body-colour of the fire-opal. The play 

 of colours of the fire-opal has a pronounced yellow or red for background, but between this 

 and yellow or red precious opal there are all possible gradations. The light reflected by 

 those fire-opals which show a play of colours is often less varied in colour than in the precious 

 •opal. The most usual tints are red and green, which in the paler-coloured stones are often 

 fine carmine-red and deep emerald-green ; a combination of yellow and blue is also met with, 

 but is much rarer. As a general rule the colours reflected by fire-opal are less brilliant, and 

 the play of colours taken as a whole is less striking than in the precious opal. 



Fire-opal while one of the most beautiful of all varieties of opal is, at the same time, 

 the least durable. Thus it may be influenced by contact with water, by sudden changes of 

 temperatm'e, or by the action of light or atmospheric conditions. Some changes in the 

 «tone are ascribed to the weather, and a fire-opal is stated to be more brilliant in summer 

 than in winter. If this is really the case, the difference is probably due to the brighter 

 light of the warmer season of the year. While some stones are more durable, others are 

 easily fractured and often lose their lustre and colour for no apparent reason, even when 

 they have been protected from external influences. Very transparent stones exhibiting a 

 play of coloui-s are specially liable to become affected in these ways, and for this reason are 

 unsuitable for use as gems. 



The disadvantages of fire-opal as a gem are, however, of little consequence, since the 

 stone, in spite of its fine appearance, is rarely met with in the trade. The price is not as 

 low as might be expected, probably because durable stones of any size are somewhat rare. 

 For a fire-opal measuring 4| lines in length and 3J in width, 1200 francs is said to have 

 been paid. 



The fire-opal is mainly a product of Mexico, and was first brought to Europe by 

 Alexander von Humboldt at the beginning of the nineteenth century. It is found in any 



