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PRECIOUS STONES.
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a. Diaphaneity.
If light passes through the mineral so that objects can be seen through
it, the mineral is said to be transparent; a mineral of a lesser degree
of diaphaneity is sub-transparent. If light is transmitted, but in such
a manner that we cannot see through the substance, the body is said to
be translucent; or sub-translucent if only a small amount of light
passes. If no light goes through the substance is opaque.
b. Refraction
is of the highest importance in the study of gem stones ; many of the
peculiarities of gems are due to this phenomenon.
1. Single refraction. All transparent minerals
which are crystallised in the cubic (or isometric) system, and all
transparent amorphous substances, are isotropic. If a ray of light A 0 (Fig. 1) impinges on a piece of glass having parallel surfaces F 0, D G, at an angle A 0 B with the normal 0 B, it is found that the ray does not pass through the glass in a straight line A 0 E, but is bent or deflected into the path 0 D, and on emerging into the air again at D it proceeds in a direction D J parallel to its original direction. The angle A O B is called the angle of incidence, and the angle DOC the angle of refraction. The lines B M and C N are proportional to the sines of these angles
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