of
heat will not fracture the stone, which upon cooling becomes white,
then green, and finally regains its original red colour. The ruby is
dichroic according to the direction in which it is seen, and in cutting
it this must be taken into consideration; the table—the largest facet
surface—should be aligned with the basal planes of the crystal, in
order to exhibit the greatest possible depth of colour. The dichroism
of the ruby is one of its certain distinctions from spinel, garnet, and
other red stones which crystallise in the cubic system and therefore
are but singly refracting.
Rubies
sometimes show on their basal planes, or on a convex surface which
corresponds to the bases, a six-rayed star of gleaming light; these are
called asteriated rubies, " star-rubies," or ruby cat's-eye.
So
valuable are flawless rubies of good colour, that when they ascend much
above a carat in sveight their prices depend to a considerable extent
on fancy. A three-carat ruby of desirable lualities is a rarity, while
three-carat diamonds ire common. Although nothing will definitely
indicate what a fine ruby of three carats and upward might bring in the
open market, yet Dr. George F. Kunz appraised a fine ruby of
