application of a
low degree of heat, it changes its colour from a yellow to a
beautiful pink. It contains fluoric acid, which may be the means
of this change.--_Mawe_.

_Why is ruby of such a brilliant colour?_

Because a sixth of it is chromic acid, while other gems, as the garnet,
are coloured by oxide of iron. The most esteemed, and at the same time,
rarest colour, of the oriental ruby, is pure carmine, or blood-red of
considerable intensity, forming, when well polished, a blaze of the most
exquisite and unrivalled tint. It is, however, more or less pale, and
mixed with blue in various proportions; hence it occurs rose-red and
reddish white, crimson, peach-blossom red, and lilac blue--the latter
variety being named oriental amethyst. A ruby perfect both in colour and
transparency, is much less common than a good diamond, and when of the
weight of three or four carats, is even more valuable than that gem. The
king of Pegu, and the monarchs of Siam and Ava, monopolize the rarest
rubies; the finest in the world is in the possession of the first of
these kings: its purity has passed into a proverb, and its worth when
compared with gold, is inestimable. The Subah of the Deccan, also, is in
possession of a prodigiously fine one, a full inch in diameter. The
princes of Europe cannot boast of any of a first rate magnitude. Mr.
Mawe, from whose interesting work we abridge these particulars,
considers the oriental sapphire to rank next in value to the ruby. Among
the British crown jewels is an inestimable sapphire; it is of the purest
and deepest azure, more than two inches long, and one inch broad. The
finest ruby among these gems is more treasured for its antiquity than
intrinsic value, it being the one worn at Cressy and Agincourt, by the
Black Prince and Henry V.: this is worn on the back cross, and the
sapphire on the front, of the imperial crown upon state occasions.

_Why are garnets often found of a reddish brown tinge?_

Because of the excess of oxide of iron which they contain; a small
proportion being sufficient to colour them entirely, without injuring
their play and splendour. In fact, the perfection of all gems depends
less on the quality of their component principles, than on their
complete solution and intimate combination. The alkalized earths, as
lime, magnesia, and still better, pot-ash, seem to intervene as
solvents, for alumina, completely dissolved, acquires, as we have shown
from Klaproth