spangles of mica. Clear, colorless pebbles of quartz are sometimes cut
for tourists. Such pebbles are frequently misnamed "diamonds" with some
prefix, as for example "Lake George diamonds," etc. Among the minutely
crystalline varieties of quartz we have the clear red, which should be
called "_carnelian_," the brownish-red "_sard_," the green
"_chrysoprase_," the leek green "_prase_," and the brighter green
"_plasma_." The last three are not so commonly seen as the first two,
and frequently the best-colored specimens are artificially dyed.

"_Jasper_," a material more highly regarded by the ancients than at
present, is mainly quartz, but contains enough earthy material to make
it opaque. "_Bloodstone_" is a greenish chalcedony with spots of red
jasper.

"_Agates_" are banded chalcedonies, the variety called "_onyx_" having
very regular bands, and the "_sardonyx_" being an onyx agate in which
some of the bands are of reddish sard.

Just as we considered opal with quartz (because of its chemical
similarity) when discussing mineral species, so we may now consider the
proper naming of opals here. "_Precious opal_" is distinguished from
"_common opal_" by the beauty of its display rather than by any
difference in composition. The effect is of course due to the existence
of thin films (probably of material of slightly different density),
filling what once were cracks in the mass. The rainbow colors are the
result of interference of light (see a college text on physics for an
explanation of interference). The varying thickness of these films gives
varying colors, so different specimens of opal show very different
effects. The differences of distribution of the films within the
material also cause variations in the effects. Hence we have hardly any
two specimens of opal that are alike.

There are, however, certain fairly definite types of opal and jewelers
should learn to apply correct names to these types. Most prominent
among the opals of to-day are the so-called "_Black opals_" from New
South Wales. These give vivid flashes of color out of seeming darkness.
In some positions the stones, as the name implies, appear blue-black or
blackish gray. By transmitted light, however, the bluish stones appear
yellow. Owing to the sharp contrast between the dark background and the
flashing spectrum colors, black opals are most attractive stones and
fine specimens command high prices. One fine piece, which was on
exhibition at the Pana