The
smaragdus of Theophrastus included with the beryl a number of quite
different stones, such as the chrysocolla and dioptase. Pliny's
smaragdus included, besides the above, the green chryso-beryl and the
chrysoprase, as well as the green plasma, the prase, and green jasper.
In native East Indian jewellery the emerald is usually cut en cabochon ;
this form conceals the flaws to a great degree. In Europe the step-cut
is considered the most suitable style. Emeralds are occasionally
engraved or carved. In the Hope collection there was a beautiful
vinaigrette made out of two emeralds, £ inch in height, and J inch
across ; it brought 145 guineas when sold by auction in 1886.
Chrysoberyl.
The
cymophane or true cat's-eye, the hard specimens called oriental
chrysolite by jewellers, and the alexandrite are varieties of
chrysoberyl. Their differences of hue and of physical appearance are
not associated with any essential differences of composition. The
colours of chrysoberyl range from columbine red through brownish yellow
to leaf green ; a golden yellow and a greenish yellow are not unusual.
The coloured chrysoberyls are stronglv dichroic ;* some brownish
specimens from this cause mav present to the unassisted eye the aspect
of tourmalines. The green leaf, or deep olive green variety, known as
alexandrite, of which fine flawless specimens of large size have been
sent from Ceylon, is remarkable for appearing of a raspberry red hue by
candle or lamplight. This mineral crystallises in the orthorhombic
system ; twins are frequent. The hardness of chrysoberyl approaches
that of the sapphire : it is 8.5. Its lustre and brilliancy are
considerable. Its specific gravity averages 3.7 ; it is but slightly
lowered by strong ignition.
Golden yellow . . 3.84 I Brownish yellow . . 3.734
Greenish yellow . . 3.70 | Alexandrite . . . 3.644
* Fig. 9. Frontispiece.
