and oxygen, in its make-up, but completely and
definitely combined in a new compound that resembles neither aluminum
nor oxygen. It is thus a compound substance. No other element than
carbon affords any gem-stone when by itself.

OXIDES OF METALS. There is, however, another oxide, in addition to
aluminum oxide, that furnishes gem material. It is _silicon oxide_,
containing the two elements silicon and oxygen. Silicon itself is a
dark, gray, crystalline element that seems half metallic, half
non-metallic in its properties. It is never found by itself in nature
but about twenty-eight per cent. of the crust of the earth is composed
of it in compound forms, and one of the most abundant of these is
QUARTZ, which is a mineral species, and which contains just silicon and
oxygen. That is, it is oxide of silicon. Now quartz is colorless when
pure (_rock crystal_), but it is frequently found colored purple
(probably by oxide of manganese) and it is then called _amethyst_ by the
jeweler. At other times its color is yellow (due to oxide of iron) and
then the jeweler is prone to call it "_topaz_," although properly
speaking that name should, as we shall soon see, be reserved for an
entirely different mineral species. _Chalcedony_ too (which when banded
furnishes us our _agates_, and when reddish our _carnelian_) is a
variety of quartz, and _prase_ is only quartz colored green by fibers of
actinolite within it.

The common _cat's-eye_ and the _tiger's-eye_ are varieties of quartz
enclosing fibrous minerals or replacing them while still keeping the
arrangement that they had. "_Venus hair stone_" is quartz containing
needle-like crystals of rutile, and "_iris_" is quartz that has been
crackled within, so as to produce rainbow colors, because of the
effects of thin layers of material. _Aventurine quartz_ (sometimes
called goldstone) has spangles of mica or of some other mineral enclosed
in it. The _jaspers_ are mainly quartz with more of earthy impurity than
the preceding stones.

Thus all this long list of stones of differing names can be classified
under the one mineral species, quartz. Together they constitute the
quartz gems. In properties they are essentially alike, having specific
gravity 2.66, hardness 7, slight double refraction, etc., the slight
differences that exist being due only to the presence of varying amounts
of foreign matter.

OPAL. The _opal_ may be considered along with the quartz gems, because,
like them, it