386 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



especially quartz and chalcedony, with which they may be associated. Common opal is so 

 widely distributed that it is scarcely possible to mention every locality. The mineral occurs 

 very abundantly in the basalts of Iceland, the Faroe Islands, the north of Ireland, the 

 neighbourhood of Steinheim, near Hanau, and many other places. It is found in trachyte 

 at a few places in the Siebengebirge on the Rhine, in the volcanic region of the north of 

 Hungary and Transylvania, in Honduras, and throughout the whole of Central America and 

 Mexico to the United States, and in many other places. It is found in serpentine in the 

 neighbourhood of Frankenstein, Silesia, in great abundance. Other modes of occurrence 

 will be incidentally mentioned below. 



Opal of all the kinds mentioned above, when cut and polished, usually acquires a good 

 lustre, and, as it is often pleasing in colour, the rounded gem-stones into which it is cut are 

 l)y no means unattractive in appearance. As, however, the rough material occurs in nature 

 in such abundance, and as, moreover, the finished product is so soft and brittle and lacking 

 in durability, these stones are always low in price and are used only in the cheapest varieties 

 of jewellery. In connection with the abundance of the mineral, it may be mentioned that 

 in former times a uniformly coloured, pale grey, translucent opal of very pleasing appearance 

 was obtained in large quantity at Steinheim, near Hanau, and was cut at Oberstein ; when 

 this deposit was exhausted, a supply of similar material was at once forthcoming from the 

 Siebengebirge on the Rhine, so that there was no opportunity for a rise in the price of this 

 variety. 



The following different kinds of common or semi-opal are sometimes used for decorative 

 and ornamental purposes : 



Milk-opal is a cloudy but highly translucent opal of a milk-white, bluish- or greenish- 

 white colour. It occurs in large amount in decomposed serpentine at Kosemiitz in the 

 neighbourhood of Frankenstein in Lower Silesia, and at other localities. Milk-opal some- 

 times exhibits black arborescent markings, or dendrites so-called, similar to those in certain 

 varieties of chalcedony (compare Fig. 89). Opal of this kind is known as moss-opal; it is 

 cut so as to bring the markings as near the surface as possible. Specially fine specimens 

 measuring 3 or 4 inches across are found in Trego County, Kansas. 



Opal-agate shows a banded structure, the bands being alternately light and dark in 

 colour, or opal and agate may be banded together in the same way. The arrangement of 

 the layers is the same as in onyx, so that, like this stone, opal-agate may be used for 

 making cameos, &c. It is found at the Gianfs Causeway, County Antrim, sometimes 

 at Steinheim, also in the Siebengebirge, and, of specially fine quality, at Guayoca in 

 Honduras. 



Prase-opal is a highly translucent opal, the beautiful apple-green colour of which is 

 due to the presence of a small amount of nickel. It occurs at Kosemiitz, near Frankenstein, 

 Silesia. 



Rose-opal is a semi-opal of a beautiful rose-red colour, probably due to the presence 

 of organic matter. It occurs interbedded with fresh-water limestone at Quincy, near Mehun 

 (dep. Cher), in France. An opal of the same character, which is cut at Oberstein, is said to 

 come from Mokiin in Upper Egypt. A variegated rose-red, yellow, and green opal of the 

 greatest beauty is found in large masses in the State of Jalisco in Mexico. 



Wax- opal is yellow in colour and is characterised by its wax-like lustre. It is 

 specially abundant in trachytic tuffs in the neighbourhood of Tokaj and Telkibanya in 

 Hungary, hence its name Telkibanya-stone. At the same place occurs also the pitch- 

 opal, a dark-brown opal with veins of a lighter colour, and with a brilliant, pitch-like 

 lustre. 



