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186 PRECIOUS STONES
6. Ruby and Tourmaline (Red)
The
red tourmaline is occasionally so nearly ruby-red in color as to
necessitate care. The dichroism of the tourmaline is very similar to
that of the ruby, but stronger. The color is not so deep nor the lustre
so great, but it is often more transparent than the ruby. The test of
hardness is sufficient, also specific gravity.
Hardness. Specific Gravity.
Ruby...................................9 3.9 to 4.1
Tourmaline.............................7.25 3.08
7. Spinel and Tourmaline (Red)
The
dichroiscope will decide between these, as the spinel is singly
refractive, the tourmaline strongly doubly refractive, and the latter
lacks any yellow tint.
8. Tourmaline (Red) and Topaz (Red)
If
the eye fails to distinguish between the rose-pink red of the
tourmaline and the denser yellowish to brownish red of the topaz,
hardness, specific gravity, or the dichroiscope will do so. One of the
twin colors of the topaz is yellow.
Hardness. Specific Gravity. Dichroism.
Topaz.........8 3.50103.56 Red and yellow.
Tourmaline... .7.25 3.08 Rose and dark red.
9. Sapphire (Blue) and Tourmaline (Blue)
These
may be distinguished by the quality of color. Whatever the shade of
color in the sapphire, from corn-flower to indigo, it is always clearly
blue. The tourmaline blue inÂclines to green. Under the dichroiscope
this is reversed, the twin colors of the sapphire being dark blue and a
light greenÂish blue, those of the tourmaline light and dark blue.
Either of the other tests is sufficient to establish identity.
Hardness. Specific Gravity.
Sapphire................................9 3.9 to 4.1
Tourmaline...... ......................7.25 3.1
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