114 
THE PRINCIPAL FLOOR. 
their excessive hardness and beauty of colour, are highly valued 
by the jeweller. These pure forms of alumina are obtained 
chiefly from Burma, Siam and Ceylon, where they occur in sands 
and gravels. The bright red varieties constitute the oriental 
ruby, so called to distinguish it from the distinct and much less 
valuable mineral, spinel, which is also known in commerce as 
“ ruby.” Sapphire is a name applied to the blue transparent 
crystals of corundum ; Pliny’s name of asteria being retained 
for the “ star sapphires,” or those varieties which exhibit a star 
of light when cut with a convex face. Sapphires of green, 
yellow and other tints are also exhibited. 
The spinel, or spinelle, of which the bright red varieties are 
used as a gem under the name of spinel ruby, may be regarded 
as a compound of alumina and magnesia, the latter being 
frequently replaced to a considerable extent by protoxide of 
iron. Alumina and glucina constitute, in like manner, the 
Chrysoberyl or Cymophane, certain varieties of which, when cut 
en cabochon, exhibit a peculiar opalescence and luminous band,,, 
whence the name oriental cat’s-eye. 
Case R — Emerald ; Tourmaline : Topaz, &c. 
The large series of minerals composed of various metallic 
silicates, in an anhydrous condition, commences in this Case and 
is continued through several of the following cases. In this 
section the visitor will probably be specially interested in such 
of the silicates as are cut and polished as ornamental stones. 
The double silicate of the allied metals aluminium and 
glucinum, or beryllium, is known in its coarser varieties as Beryl , 
of which enormous six-sided crystals are found in the United 
States. The fine transparent green varieties are distinguished 
as Emerald, while those of paler tint pass under the name of 
Aquamarine. The rich colour of the emerald is due either to 
oxide of chromium, or to an organic colouring matter. The 
gem is chiefly obtained from the mines of Muzo in Colombia, 
where it occurs crystallised in a black limestones of cretaceous 
age. Specimens will be found showing its occurrence in the 
matrix, accompanied by some fossils from the limestone. 
Closely related to the emerald in chemical composition is the- 
mineral called Euclase, the excessive brittleness of which renders 
it useless for purposes of ornament. The American beryllonite , 
a phosphate of beryllium and sodium, and the Russian phenakite, 
a silicate of beryllium, are here exhibited both in the rough 
state and cut as gem stones. 
Few minerals present greater complexity and variability of 
composition than the Tourmaline, to which we next pass. Its 
crystals are remarkable for a want of symmetry between the 
opposite ends, and for acquiring electric properties on exposure 
to heat. The black tourmaline is commonly known as Schorl, 
and the pink as Rnbellite ; a striking variation of tint in the 
