ly ruby undergoes a series of color changes on
being heated, but returns through the same series in reverse order on
being cooled, and finally resumes its original color. Strong heating
will whiten some yellow sapphire. The author thus obtained a white
sapphire from a crystal of light yellow material.

It is interesting to note that the corundum gems undergo marked change
in color under the influence of radium. A regular series of changes is
said to be produced in white sapphire by this means, the final color
being yellow. This color may then be removed by heat and the series run
through again. It is not stated that a fine red has ever been thus
obtained. Perhaps Nature, by her slower methods, using the faint traces
of radio-active material in the rocks, reddens the corundum of Burmah at
her leisure, and finally arrives at the much sought "pigeon blood"
color. It is said that the natives of India have a legend to the effect
that the white sapphires of the mines are "ripening rubies," and that
one day they will mature. Perhaps they are not far wrong.

DIAMOND. Diamonds of yellowish tint may be improved in color by the use
of high-power radium. At present the latter is so rare and costly that
there is no evidence of its commercial use for this purpose. Scientists
have brought about the change to a light blue as an experiment. It is
not yet known whether the change will be permanent. Perhaps here again
Nature has anticipated man's discovery and made the fine bluish-violet
Brazilian diamonds (which fluoresce to a deep violet under an arc light,
and which shine for a few moments in the dark after exposure to light)
by associating them for ages with radio-active material. Some of the
African stones also have these characteristics.

Aside from the change in the color of diamond that may be brought about
by means of radium, the mineral is extremely reluctant to alter its
color. Many experimenters besides the author have tried in vain a host
of expedients in the hope of finding some way to improve the color of
diamond. About the only noticeable alteration that the author has been
able to bring about was upon a brown diamond, the color of which was
made somewhat lighter and more ashen by heating it in a current of
hydrogen gas to a low red heat.

LESSON XXVII

PEARLS

Unlike the gems that have been so far considered, the pearl is not a
mineral, but is of organic origin, that is, it is the product of a
living organi