FIKST FLOOK. 
57 
ease with which it is worked and takes a good polish, its green General 
colour, and varied markings render it much sought for as a ^^^rals? 
material for fire-places, tables, and other indoor work : exposed 
to the weather it soon loses its polish. Only specimens illustrat- 
ing the purer forms of the mineral are shown in the case. 
Topaz (25d) in its clear varieties is one of the precious stones. 
The crystals from the Urulga- river, in Siberia, are remarkably 
fine examples of crystalline development ; they are of a delicate 
brown colour, but are kept covered up, as the action of light 
speedily bleaches them. The yellow crystals from Brazil assume 
a peculiar pink colour when heated, and are then known to 
jewellers as Burnt or Pink Topaz. 
Garnet also belongs to the group of precious stones ; when 
the red is tinged with violet, it is the Almandine and the Syrian 
garnet (from Syriam in Pegu), and when cut en cahochon, the 
Carbuncle of jewellery (26f ); the Cinnamon-stone or Essonite 
is yellow (26e) ; the Pyrope and the Bohemian garnet are 
blood-red (26e) ; Uvarovite is a green chrome-garnet (26h). 
Jadeite (27a) is one of the green stones, which, under the 
name of jade, are wrought into ornaments in China : from jade, 
however, it is distinguished by its chemical composition, struc- 
ture, and higher specific gravity. 
Among the specimens of Epidote (27c) a remarkable suite 
from the Untersulzbachthal is exhibited. 
Mica (28a) is the name given to a group of minerals differing 
much from each other in chemical composition and optical pro- 
perties, but having as a common character an easy cleavage in a 
single direction, and thus affording plates remarkably thin, 
transparent, tough, and elastic. One of these minerals, musco- 
vite (28d), has been used in Eussia in place of glass for win- 
dows ; it is now in common use for lanterns and stoves, not 
being so easily cracked as glass by change of temperature : it 
is still known in commerce as talc, a term formerly applied to 
it by mineralogists, but now restricted by them to a different 
mineral. 
The group of Felspaxs, 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 
