30 MR. J. T. HOTBLACK ON PRECIOUS STONES. 
valuable, as is also the reddish or fire opal ; when less perfect it is 
the semi or common opal. The noble or precious opal is one of 
the most beautiful gems in existence. The “ opalescence ” is due 
to the presence of the water in the silica. Opals are always cut 
en cabochon on both sides, they are very brittle. Opals are much 
more brilliant when in a warm atmosphere, and dealers, knowing 
this, are in the habit of holding them in the hand before showing. 
Fine stones of large size are very rare, they seldom exceed an inch 
in diameter. For large fine specimens as much as £1,000 has been 
frequently paid. The finest known is in the Museum at Vienna, 
it came from the celebrated mines at Carnowitza, which have been 
worked since 1400; it is 4f ins. by 2£ ins., weighs 17 ozs., and 
has been valued at £70,000. 
The hydrophane or Mexican opal quite loses its beauty if allowed 
to get damp or wet, and Sir Walter Scott, knowing this fact, makes 
use of it in ‘ Anne of Geierstein,’ after the publication of which, 
the belief that opals were unlucky became so general that they 
went very much out of fashion. The original splendour can, 
however, be quite restored by the application of a little heat. Of 
late years opals have been again coming in fashion, and they should 
be very highly esteemed, not only for their natural beauty, but also 
because they are, perhaps, the only precious stones which cannot 
be imitated. 
Wood-opal. 
Having described amber and jet, I think it right to include 
wood-opal, which is also of organic origin, and but very distantly 
related to true opal. Wood opal is silicified wood, or wood petrified 
by silica, and it generally retains the structure of the original wood. 
It is found in many places, but, perhaps, most abundantly in the 
desert between Cairo and Suez, where silicified trunks of trees 
belonging to the Nicolia cegyptiaca sometimes 40 or 50 feet long, 
and one or two feet in diameter, are found lying in all directions. 
Wood-opal has considerable beauty, and is used for several artistic 
and ornamental purposes. 
Obsidian. 
Obsidian is a volcanic glass found in many lavas ; it has 
