OPAL (HYDROPHANE) 387 



"Wood-opal of a paler or darker colour arises from the opalisation of fossil wood. 

 The structure of the wood down to the minutest detail is often to be seen in the polished 

 surface of the opal, giving it a curious appearance. This variety occurs in large amount at 

 the Hungarian locality just named, in Tasmania, in the Siebengebirge, and at many other 

 places. 



Menilite occurs as greyish-brown, rounded nodules in clayey shale at Menilmontant 

 and St. Ouen, near Paris. When polished it acquires a brilliant lustre, and those stones 

 which exhibit alternate bands of grey and brown are decidedly pretty objects. 



Hydrophane is an opal which may be dirty white, yellowish, brownish, reddish, or 

 greenish in colour, and which in its natural condition has little lustre and translucency. In 

 mass it is almost opaque, and very little light passes through even the thinnest of splinters. 

 Hydrophane possesses, however, one very remarkable property on which depends its 

 ■occasional application as a gem. By the absorption of water it becomes almost perfectly 

 transparent ; some specimens even acquire the play of colours characteristic of precious 

 •opal, and are then known as oculus mundi. The capacity of hydrophane for absorbing 

 large quantities of water is due to the great porosity of the substance ; so eagerly does 

 it suck up water that it will adhere to the tongue ; moreover, its immersion in water 

 is often accompanied by a hissing sound due to the rapid expulsion of bubbles of air. 

 The transparency of hydrophane, acquired in the way described, is not permanent, 

 however, and on drying the stone gradually becomes again cloudy and opaque, any 

 play of colours it may have acquired being lost. So long as the water in which the 

 stone is immersed is pure the phenomenon may be repeatedly observed. Hydrophane is 

 sometimes used as a gem, and when this is the case it is cut with a rounded surface in the 

 form of a lenticle and set A jour in rings or as a pin, so that there is nothing to prevent the 

 stone being immersed in water at will and its peculiar property exhibited. It is not 

 surprising that the behaviour of hydrophane, under the circumstances described, inspires 

 considerable awe and wonder in the minds of Eastern people, and especially of the natives of 

 Java and other East Indian islands, by whom the stone is much worn as an amulet. It is 

 .said that a large number of stones are every year exported from Europe, and especially from 

 Oberstein, to these islands and there sold to the natives. 



The transparency acquired by hydrophane after immersion in water is very fleeting ; a 

 more permanent effect, lasting perhaps as long as a year, is obtained by placing the stone in 

 hot oil. A somewhat different effect again is obtained by allowing the porous mass to 

 become impregnated with pure wax or spermaceti ; the stone is then cloudy when cold, but 

 when slightly warmed and the wax melted it assumes a brown or grey colour and becomes 

 highly translucent or almost transparent. For this reason the mineral is sometimes known 

 as pyrophane. It can be coloured by immersion in coloured liquids, and it is said that in 

 former times it was brought into the market dyed red or purple. 



Hydrophane is not particularly abundant, and as there is a certain appreciable demand 

 for it it commands a fair price, the value of any given stone depending upon the size of the 

 stone and the degree of transparency or play of colours it acquires when placed in water. 

 The most important locality is probably Hubertusburg in Saxony ; the mineral occurs here 

 in a porphyry, either as thin strings or in nodules of chalcedony, with amethyst, rock-crystal, 

 and common opal. When found in the mother-rock the siliceous masses are often still 

 soft and gelatinous, the material gradually assuming the characters of hydrophane as 

 it dries up on exposure to the air. It also occurs with the precious opal of Hungary, 

 with the fire-opal of Mexico, with the various kinds of opal found in the Faroe Islands 

 and Iceland, and at some other localities where opal is found, but always sparingly 



