1874.] Beryls and Emeralds. 505 
gravity is but 2°652 at the most, while all beryls, when free 
from any kind of imperfe¢tions, such as cavities, fissures, 
&c., appear to range in density from 2°69 to 2°705, and con- 
sequently sink. The same is true of the emerald when 
perfect, but the general presence of internal flaws in this 
stone, even though they may be very minute, lowers its spe- 
cific gravity decidedly, and may even reduce it to that of 
rock crystal. It should be stated here that the specific 
gravity of beryl is not altered by any temperature short of 
that necessary to effect the fusion of the stone, and thereby 
to change its crystalline structure to the condition of a 
glass. 
A word concerning the hardness of beryl will suffice. 
Pure, transparent, and colourless or pale specimens are 
harder than quartz, varying from 7°5 to nearly 8; yet the 
best emeralds—though they scratch quartz, and of course 
all kinds of glass and paste as well—are slightly softer than 
the paler varieties of the species, such as the aquamarine. 
We now come to the question of the colour of the 
emerald, a question which has been lately studied with par- 
ticular care by Mr. Greville Williams. The green hue of 
this gem is of a different quality from that of the precious 
beryl and the aquamarine,—indeed it closely approaches, in 
really good specimens, to the pure green of the solar 
spectrum. The colour is, also, far more permanent than 
that of some beryls, especially those of a yellow or greenish- 
yellow hue, which become bluish, or even pale blue, when 
heated before an ordinary gas blowpipe jet, long before they 
show any signs of incipient fusion. The emerald, on the 
other hand, when heated alone before the oxyhydrogen blow- 
pipe, bears a bright red heat without loss of colour, and 
even when it begins to fuse the edges only become colourless 
and opaque, the centre remaining green. Even when fused, 
emeralds are kept at the maximum temperature attainable 
by means of the oxyhydrogen blowpipe, they retain an 
opalescent dull green hue for some time, and it is only after 
prolonged heating that they become at last transparent, and 
almost free from colour. Greville Williams has also found 
that the addition of chromium sesquioxide to a fused 
emerald bead which has become colourless produces only a 
dull green colour, or, at the best, a colour decidedly inferior 
to that of the emerald. Cobalt oxide gives a fine blue tint 
when similarly added to an artificial mixture having the 
composition of the natural beryl or emerald, and this blue 
colour is not altered by the high temperature of the oxy- 
hydrogen flame. With such artificial beryl composition 
