376 SYSTEMATIC DESCRIPTIOxN OF PRECIOUS STONES 



The value of an opal depends in the first place upon the brilliancy and beauty of the 

 colours it displays, and in the second place upon the uniformity with which the play of 

 colours is distributed over the surface of the stone. The stones which show a brilliant play 

 of colours over the whole surface are most valuable, while in inferior stones the colours are 

 dull and there are portions of the surface which show no play of colour. 



When precious opal is exposed to the action of heat it loses its constituent water ; the 

 play of colours then disappears and the stone becomes cloudy. Some stones indeed slowly 

 lose water at ordinary temperatures, thus suffering a gradual diminution in beauty and 

 value. It is stated that in such cases the play of colours may be restored by immersing the 

 stones in oil, but that, as the oil gradually decomposes, the stone becomes poorer and poorer 

 in appearance until at last the play of colours completely disappears, and the stone itself 

 assumes a dirty brown tinge. Hungarian precious opal is least subject to such changes, and 

 for this reason is worth more than that from other localities. 



The substance of the precious opal is in itself colourless, and the brilliant play of 

 colours so characteristic of this gem is due purely to changes effected in the incident rays 

 of light during their passage through the substance of the opal, probably in a way which 

 does not differ essentially from that whereby the iridescence of certain specimens of quartz 

 and of other minerals arises. The drying up and solidification of the gelatinous silica 

 would be attended by the development of a network of cracks and fissures ; these cracks, by 

 their action on the rays of light at their surfaces, give rise to a display of the rainbow 

 colours characteristic of thin plates. Microscopical examination has demonstrated that 

 these cracks are sometimes lined with a film of opal, the refractive index of which differs 

 from that of the main mass. It has therefore been conjectured that the play of colours 

 of the opal may be due in part to this or to some other circumstance, seeing that it 

 is so much more brilliant and magnificent in this stone than in any other iridescent 

 mineral. It is indeed possible that the phenomenon in opal has not as yet been 

 completely explained by any of the various theories which from time to time have been 

 promulgated. There can be absolute certainty only on one point, namely, that the colours 

 of the precious opal are effects of the interference of light and not of an admixture of 

 pigment, since they are pale or completely absent when the stone is viewed by transmitted 

 light. 



On account of its much fissured condition, precious opal, although perhaps slightly 

 harder, requires more care in handling than does common opal. Sudden and extreme 

 changes in temperature must in particular be guarded against, in order to avoid the 

 fragmentation of the stone. For the same reason, special care is necessary during the 

 processes of cutting and grinding. 



Precious opal is in almost all cases cut in a rounded form, for, not only does the 

 existence of facets if anything detract from the colour effect of the stone, but owing to its 

 softness the edges between the facets would very soon lose their sharpness. Opal is, 

 therefore, but rarely cut in faceted forms, though the table-cut and the step-cut are some- 

 times to be met with. According to the form of the rough stone, a cut opal may be 

 circular or oval in outline and moi'e or less convex, so that it may resemble the half of a pea, 

 a bean, or an almond. It is ever the aim of the lapidary to perform the operation of cutting 

 with as little waste of the valuable material as possible, while at the same time he 

 must contrive so that the play of colours is displayed to the best advantage. This not 

 infrequently requires great skill and much thought and consideration ; the removal of 

 the matrix and of those portions of the stone which show no play of colours with the least 

 possible waste of precious material, and in such a manner as to ensure an uninterrupted 



