PRECIOUS STONES 187
10. Cyanite, Fluorite, and Iolite
These
stones, when blue, may be known by the several tests, and by certain
peculiarities which characterize them. The lustre of the cyanite
usually inclines to pearly; that of the iolite is somewhat greasy. The
fluorite is generally phosphorescent at low temperatures.
Hardness. Specific Gravity.
Cyanite ... 5 to 7 3.60 Dichroism perceptible.
Fluorite... 4 3.1 to 3.2 Single refraction.
Iolite......7.25 2.60 to 2.66 Dichroism, blue and yellowish gray.
11. Sapphire, Topaz, Aquamarine, and Tourmaline, in
Light Blue
The
identity of these stones is easily determined. They are all doubly
refractive, but the twin colors of the topaz only are distinct. They
are colorless and bluish. Sapphire has a harder lustre; tourmaline a
more inky color; aquamarine is a water blue; and topaz may be known by the hardness and specific gravity.
Hardness. Specific Gravity.
Sapphire...............................9 3.9 to 4.1
Topaz.................................8 3.50 to 3.56
Aquamarine............................7.75 2.68 to 2.75
Tourmaline............................7.25 3.1
12. Sea-Green Sapphire, Topaz, Aquamarine, and Euclase
These
are all doubly refractive, but as the euclase is very rare, and the
sapphire in this color is not often seen or desired, question is most
likely to arise between the topaz and the aquamarine. In appearance
they are very much alike. The dichroisms are distinct, however, the
twin colors of the topaz being colorless and greenish blue, those of
the aquamarine bluish and yellowish, and of the sapphire and euclase
indistinct.
