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PRECIOUS STONES
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" Translucent," when objects cannot be seen, though light is transmitted.
" Subtranslucent," when the edges only transmit light faintly.
" Opaque," when no light is transmitted.
It should be noted, however, that objects cannot be seen through transparent stones when cut en cabochon. That is not owing to lack of diaphaneity, but to optical phenomena due to the convex shape.
"
Cleavage" refers to the property of separating or splitting on certain
lines into natural layers. When " perfect," the split is clear and
even, leaving planes of equal lustre. If " imperfect," the cleavage
planes are irregular.
"
Fracture" means the breaking of a stone other than on the lines of
cleavage. " Conchoidal" means that it breaks with a curved or concave
and convex surface, " even" when the surface of the break is flat or
nearly so, " uneven" when the surface is irregular.
When a stone is said to be " brittle," it means that parts of it separate in powder on attempting to cut it.
" Under the blow-pipe" means, unless otherwise specified, without the assistance of a flux.
" Amorphous," from the Greek, meaning " without shape," refers to a condition of minerals having no regular structure or form.
" Reniform," kidney-shaped.
" Botryoidal," a surface consisting of a group of rounded prominences.
"
Streak" is the color of the surface of a stone where it has been rubbed
or scratched. The powder abraded is the " streak-powder."
" Play of colors" is the prismatic effect produced by dispersion of the rays of light, as by the diamond and precious opal.
" Change of colors" occurs in stones like the labradorite, where the colors change as the stone is turned.
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