ess of color.
When it remotely suggests emerald it is called "Oriental" emerald to
denote that it is a corundum gem. Most green sapphires are of too blue a
green to resemble emerald. Some are really "Oriental" aquamarines. In
some cases the green of the green sapphire is due to the presence,
within the cut stone, of both blue and yellow portions, the light from
which, being blended by its reflection within the stone, emerges as
green as seen by the unaided eye, which cannot analyze colors. The dark
sapphires of Australia are frequently green when cut in one direction
and deep blue when cut in the opposite direction. The green, however,
is seldom pleasing.

7. Chrysoberyl as usually seen is of a yellowish green. The fine gem
chrysoberyls known as alexandrites, however, have a pleasing bluish
green or deep olive green color by daylight and change in a most
surprising fashion by artificial light under which they show raspberry
red tints. This change, according to G. F. Herbert-Smith, is due
principally to the fact that the balance in the spectrum of light
transmitted by the stone is so delicate that when a light, rich in short
wave lengths, falls upon it the blue green effect is evident, whereas
when the light is rich in long wave lengths (red end of the spectrum),
the whole stone appears red. The strong dichroism of the species also
aids this contrast. The chrysoberyls of the cat's-eye type (of fibrous
or tubular internal structure) are usually of olive green or
brownish-green shades.

Those who wish to further study color distinctions in green stones are
recommended to see the chapters on beryl (pp. 184-196), peridot (pp.
225-227), corundum (pp. 172-183), tourmaline (pp. 219-224), chrysoberyl
(pp. 233-237), and garnet (demantoid, pp. 216-218) in G. F.
Herbert-Smith's _Gem-Stones_.

LESSON XIII

COLOR--_Continued_

BLUE STONES

The species that furnish blue stones in sufficient number to deserve
consideration are, aside from opaque stones:

1. Corundum (sapphire).

2. Spinel.

3. Tourmaline.

4. Topaz.

5. Diamond.

6. Zircon.

1. Of these minerals the only species that furnishes a fine, deep
velvety blue stone is the corundum, and fine specimens of the cornflower
blue variety are very much in demand and command high prices. The color
in sapphires ranges from a pale watery blue through deeper shades (often
tinged with green) to the rich velvety cornflower blue that is so