506 Beryls and Emeralds. [October, 
other coloured glasses may be made, one of the most inte- 
resting of these being that containing didymium oxide, 
which imparts a.lively pink colour to the fused bead. 
Moreover, this beryl glass shows the black absorption spec- 
trum of didymium in a very perfect manner. The artificial 
beryl-glasses were made by taking the essential constituents 
of the mineral in the percentage proportions here given :— 
DALICALS inui nacgh) Go eel gehen Bee 
ALUMINA ¢. st Gah octets Sete 
Giueinar, 05) hs.) a9 “ovaghie eens 
I00°O 
By the fusion of these ingredients together a beryl-glass is 
obtained which is pra¢tically identical in physical and 
chemical charaé¢ters with that resulting from the fusion of 
the native emerald or beryl. In each case the hardness is 
reduced below 7 (that of quartz), and the specific gravity is 
only 2°42 instead of 2°7. These are signs that the com- 
pound has passed into the vitreous condition, and that it is 
destitute of crystalline structure. Though really identical 
in chemical composition with the native beryl, these beads 
are nothing more than “ beryl-glasses,” and can never in 
such a condition compete with the true native stones. Even 
were the exact tint of colour to be achieved, and if the 
hardness and specific gravity of the artificial glasses were 
much increased, still there would remain one fatal inferiority 
to the true emerald. For an emerald is dichroic, exhibiting, 
when viewed along the principal axis of the crystal, a green 
hue decidedly dashed with yellow, while when viewed in any 
direction transverse to this it shows a green colour verging 
a little towards blue. The instrument known as Haidinger’s 
‘*‘dichroiscope”’ enables these two pencils, of differently 
coloured (and differently polarised) rays, to be seen alongside 
of each other if a beam of white light be transmitted 
through the crystal along one of the secondary axes. Now 
the crystalline structure upon which this dichroism depends 
has not yet been imitated in the artificial emerald, and in 
consequence the play or fluctuation of three hues—the 
normal emerald-green, the yellowish-green, and the bluish- 
green—of the natural crystal must be wanting in any 
vitreous imitation of it. This dichroism of the best emeralds 
is often seen when a number of small gems have been cut 
from a single crystal of apparently uniform colour; some of 
the polished pieces, having been cut in a different direction 
with regard to the axes of the crystal, will show decided 
