PRECIOUS OPAL 375 



magnificent play of delicate colours seen on its surface, a feature which is to be found in no 

 other stone, so that in this respect opal is unique. 



Precious opal, as a general rule, is translucent or at most semi-transparent, a greater 

 degree of transparency being exceptional. When seen by transmitted light the stone 

 appears reddish-yellow in colour, but in reflected light it is colourless with a milky cloudiness, 

 or milk-white with a faint shade of blue or pearl-grey. A pronounced body-colour, such as 

 yellow, red, blue, green, or black, is very rarely seen. Of these, yellow, ranging from wine- 

 yellow to sulphur-yellow, and red, especially a yellowish-red shade, are less infrequent than 

 others. Rose-red opal, of which a magnificent example is preserved in the " Green Vaults " 

 of Dresden, is very rare. Black opal is also rare and is sometimes of very great beauty, the 

 play of colours showing up with striking effect against the dark background of the stone. 



The play of colours characteristic of all precious opal is only shown when the stone is 

 seen by reflected light ; in transmitted light it is completely absent. The display of colour 

 may be visible over the whole surface of the stone, or may be limited to isolated spots which 

 merge imperceptibly into the surrounding uncoloured portions. Again, the whole surface 

 may show a play of one uniform colour, yellow and green being in such cases much admired. 

 In other stones there may be areas over each of which there is a play of a single colour, the 

 play of colour over any one area diffieriug from that over any other, and the difierent 

 areas merging gradually into each other. Moreover, in some opals minute spangles of 

 various colours are distributed in large numbers over the surface of the stone, giving a 

 variegated kaleidoscopic effect, which has been compared to the iridescence of the neck- 

 plumage of some pigeons or of a peacock's feather. The opals of which the general colour 

 eff'ect is more uniform resemble mother-of-pearl more closely. The colour of precious opal 

 of good quality is always, however, more fiery than that of any of the objects to which it 

 has been compared. An attempt at the reproduction of the colour eff'ect of a few precious 

 opals is given in Figs. 6 to 9 of Plate XVI. 



In attempting a word-picture of the appearance of precious opal one cannot do better 

 than quote Pliny, who described this stone as combining in itself the fiery red of the ruby^ 

 the magnificent green of the emerald, the golden yellow of the topaz, the deep blue of the 

 sapphire, and the rich violet of the amethyst. All these colours may, as a matter of fact, 

 be detected in one and the same stone ; it often happens, however, that a few only are 

 present, while some stones display but one. The play of colours is indeed very variable and 

 never identical in any two stones, especially those from different localities. Thus, for 

 example, in the Hungarian opal the colour is distributed irregularly in small patches and 

 spangles giving a variegated effect (Plate XVI., Figs. 8 and 9), while in the Australian opal 

 large areas of the surface display one uniform colour (Plate XVI., Figs. 6 and 7). 



A number of varieties of precious opal are recognised, the distinction between them 

 being based on differences displayed in the play of colours. Those stones in which close-set, 

 angular patches of colour form a minute variegated mosaic, as it were, are known as 

 harlequin-opal, a term which is also applied sometimes to stones in which the ordinary play 

 of colours has a yellowish-red instead of a white background. In &jlame-opal the colours 

 are distributed more or less regularly in bands and streaks, while the whole surface of a 

 gold-opal glows with golden light. The name girasol is sometimes applied to an almost 

 transparent opal, over the surface of which there travels, as the stone is moved about, a 

 wave of blue light; the same term is, however, also applied to other precious stones. 

 Opal-onyx is built up of alternate layers of precious and of common opal. Other varieties 

 also are distinguished, some of which will be mentioned under the various localities at which 

 they are found. 



