98 PRECIOUS STONES
worn,
irregular, or fractured pieces free of matrix. They are found in a
brownish-yellow clay called " byon," widely distributed over the
valleys among the hills, or under the lower slopes of the mountains,
where they were doubtless carried by torrents from the disintegrated
matrix.
The
ruby matrix is a species of limestone associated with basic rock, and
is supposed to have been produced by the alteration of lime-bearing
feldspars from which carbonate of lime was eventually formed, while the
aluminum silicates by decomposition produced alumina. This later, under
heat and pressure, crystallized as corundum and became ruby or sapphire
according to the conditions during crystallization.
The
color of rubies is supposed to be due to the presence of a small
quantity of oxide of chromium, though experiments made in the efforts
to produce them artificially show that, although oxide of chromium
usually gives the crystal a red color, under some conditions it
produces blue, and under others both red and blue, in the same crystal.
Although
more even than that of the sapphire, the color of the ruby is rarely
distributed quite evenly through the stone. Thin white streaks often
lie within the body of the red; sometimes these disappear by the
application of heat. It can be subjected to a high temperature if the
heat is applied gradually. In cooling it becomes first white, then
greenish, and finally red again. Heat does'not destroy or permanently
change the color.
It
has been found by experiments of M. Chaumet that Burmah rubies are
extremely sensitive to violet light, which excites on them intense
fluorescence, whereas it produces but feeble fluorescence on Siam
rubies.
The
best color of a ruby is developed by cutting the table of the stone
parallel to the end facets of the crystal, and the weakest by cutting
it parallel to the prism facets.
One of the most valuable characteristics of the ruby is that the color is equally beautiful by day or artificial light.
