OPAL: COMMON VARIETIES 385 



amount here, only in a porphyritic trachyte at Villa Seca, near Zimapan, in the State of 

 Hidalgo, a little to the east of Queretaro and north of the city of Mexico, in latitude 20° 

 44|' N. and longitude 81° 411' W. of Greenwich. Together with common opal it fills the 

 cracks and crevices of the mother-rock, and occurs also as isolated masses of larger size, the 

 colour of which varies in the way described above. Many of these opal masses are invested 

 with a layer of snow-white, greyish, or brownish porous material of greater or less thickness, 

 due to the weathering of the stone (Plate XVI., Fig. 10). Besides the locality near 

 Zimapan, the fire-opal is found near Tolima in Mexico, in Honduras, at a few places in 

 North America, in the Faroe Islands, and elsewhere, always, however, together with other 

 kinds of opal and under essentially the same conditions as at Zimapan. All these 

 occurrences compared with that of Zimapan are of little importance, and to the trader in 

 ])recious stones of none whatever, so that they require no further consideration. 



OTHER VARIETIES OF OPAL (COMMON OPAL, SEMI-OPAL, &c.). 



The other vai-ieties of opal are not comparable in beauty of appearance either with 

 precious opal or with fire-opal. They are used occasionally in cheap jewellery, but find a 

 more extensive application in the manufacture of fancy goods, such, for example, as the 

 knobs of umbrellas and sticks, for snuff-boxes, seals, knife-handles, &c. ; they will therefore 

 receive here only a brief consideration. 



One of the varieties of opal which show no play of colours is sometimes perfectly 

 transparent, and when this is the case is either perfectly colourless and water-clear, or tinged 

 slightly with red or brown. It occurs as a secondary formation in the crevices of basalt 

 and other rocks containing silica, in the form of thinner or thicker crusts with a botryoidal 

 surface. From its glassy aspect it is known as hyalite, glass-opal, or Miiller's glass ; it is the 

 purest and clearest variety of opal, but is rarely cut as a gem. Opal intermediate in 

 character between hyalite and cornmon or semi-opal also occurs ; it is neither as clear nor 

 as colourless as hyalite, having a faint bluish or yellowish tinge and a slight milky 

 cloudiness. 



Opal in its purest condition is water-clear ; the presence of impurities of various kinds 

 causes it to lose its transparency', colourlessness, and some of its lustre. Different specimens of 

 opal may thus exhibit great diversity in appearance, while preserving unaltered the 

 characters typical of the species. It is on such differences in transparency, colour, and 

 lustre that the distinction between the varieties recognised by mineralogists is based. There 

 is no sharp separation between these varieties, and, specimens intermediate in character are 

 always to be met with. Common opal is translucent, and, as a rule, only slightly coloured. 

 Semi-opal is less translucent, and ranges from colourless to deeply coloured. Opal-jasper or 

 jasper-opal is very slightly translucent, and by reason of the large amount of impurities, 

 especially of ferruginous material, which is present, is deeply coloured — reddish-brown, yellow, 

 and different shades between green and black being met with. The usual vitreous lustre of 

 opals is sometimes replaced by a greiisy lustre which may incline to the waxy, the pitchy, 

 or the resinous type.\ Yellow opal with a waxy lustre is known as wax-opal, brown opal with 

 a pitchy lustre as pitch-opal, and opal with a resinous lustre as resin-opal. Wood, when 

 silicified, furnishes another variety of opal known as wood-opal, and there are others which 

 need not now be enumerated. 



These different kinds of opal occur for the most part in the manner described above, 

 and are associated together in large masses. The dift'erent kinds often occur in layers, or 

 are otherwise regularly arranged with respect to each other and to other siliceous minerals, 



2ii 



