PRECIOUS STONES. 191
cut
in steps. Since the dispersive power is small there is no marked play
of colour, and hence there is not the same importance in giving the gem
an exact form as in the case of the Diamond. Hence, too, rose cut
Eubies are relatively more effective than rose cut Diamonds.
The
grinding is now usually effected with Diamond dust on account of the
quicker abrasion ; the wheel used is an iron one. Polishing is done on
a copper disc, dressed with tripolite.
Ruby
ranks above Diamond in point of value for good stones ; while the price
of a pale Ruby of one carat may only be £1, a stone of rich deep
colour, weighing when cut one carat, may fetch £25 or more. Mr.
Streeter states that £20,000 has been paid for a very fine Ruby of 38 9/16 carats.
While Rubies up to 2,000 carats have been found, most of the larger
ones show considerable imperfect areas, and of large flawless Rubies
very few are known compared to Diamonds of similar quality and size.
On
account of the great demand for this rare gem, stones of inferior kind
are not infrequently offered as Rubies. True Rubies, although they can
be produced artificially (see " Artificial Production "), cannot be
made of sufficient size yet for gem use, and moreover the process is a
very expensive one. The most common substitutes for Ruby are the Balas
Ruby (red Spinel) and the Rock or Elie Ruby (Garnet). Both these
minerals are cubic in their crystalline form, hence the optical
properties can readily be used to distinguish them, for they show no
double refraction and no dichroism. Their hardness is in both cases
inferior to that of Ruby and the specific gravity lower. Two other
substitutes, however, belong to the same crystallographic system ;
