92 
MINERAL GALLERY. 
much from each other in chemical composition and optical pro- 
perties, but having as a common character an easy splitting, or 
cleavage, in a single direction, and thus affording plates re- 
markably thin, transparent, tough, and elastic. One of these 
minerals, Muscovite (28d), has been used in Eussia in place of 
glass for windows, and is now in common use for lanterns and 
stoves, not being so easily cracked as glass by changes of 
temperature ; is often known in commerce as talc, a term 
restricted by mineralogists to a different mineral. 
The group of Felspars, the most important of the rock-forming 
minerals, begins at case 28f. After the Felspars comes Beryl, 
of which the bright green variety. Emerald (29c), is one of the 
most valued of precious stones. It was in ancient times 
worked in Egypt, as is proved by specimens found in the 
old workings by Sir Gardner Wilkinson, and presented by 
him to the Museum. Emeralds occur in the Urals ; but the 
locality for the finest stones has long been Muso, about 
seventy miles from Santa Fe de Bogota, in South America. 
Emeralds, though not of a good colour, also occur at Stony 
Point, North Carolina. Facetted specimens of the colourless 
beryl, and also of the bluish-green beryl, known in jewellery as 
Aquamarine, are exhibited (30a). 
In cases 30e to 32d will be found examples of the Zeolite 
group of minerals. 
Tourmaline (33a), when free from flaws, is, in some of its 
varieties, to be classed with the precious stones. Among these 
is the pink variety called Eubellite. Two fine specimens of 
rubellite from Burma are shown in the case ; one, remarkable for 
its size and shape, was brought from that country by Colonel 
Symes, to whom it had been presented by the king; the 
other, not so large but of a deeper colour, was presented 
to the Museum by Mr. C. S. J. L. Guthrie. The pink- 
and-green tourmalines from Maine, U.S.A., are among the 
more beautiful of the mineral products of the United States. 
Examples of the blue tourmaline, Indicolite, are shown in 
case 33b. 
A rich blue mineral, the Lapis-Lazuli of jewellery (34b), 
brought from Persia, China, Siberia, Bokhara, and Chili, 
is a mixture of various species. When powdered, lapis- 
