of
magnesium.

ZIRCON is itself a species of mineral and is a silicate of zirconium.
The names _hyacinth_, _jacinth_, and _jargoon_ are applied to red,
yellow, and colorless zircon in the order as given.

JADE may be of any of several different species of minerals, all of
which are very tough. The principal jades belong, however, to one or the
other of two species, _jadeite_ and _nephrite_. Jadeite is a sodium
aluminum silicate and nephrite, a calcium magnesium silicate.

Leaving the silicates we find very few gem minerals remaining. The
phosphates furnish us _turquoise_, a hydrous aluminum phosphate, with
copper and iron. _Variscite_ is also a phosphate (a hydrated aluminum
phosphate).

The carbonates give us _malachite_ and _azurite_, both carbonates of
copper with combined water, the malachite having more water.

LESSON XIX

THE NAMING OF PRECIOUS STONES

Owing to the confusion which may result from a lack of uniformity in the
naming of precious stones, it is very desirable that jewelers and stone
merchants inform themselves in regard to the correct use of the names of
the gems, and that they use care in speaking and in writing such names.

As nearly all precious and semi-precious stones are derived from a
relatively small number of _mineral species_, as we saw in Lesson
XVIII., and as the science of _mineralogy_ has a very orderly and
systematic method of naming the minerals, the best results are had in
the naming of gems when we use, as far as is possible, the language of
mineralogy.

ANCIENT USAGE. Long established custom and usage, however, must be
observed, for any system of naming must be generally understood in order
to be useful. Thus the proper name for blood red, crystallized oxide of
aluminum, of gem quality, according to the mineralogical system of
naming, would be red _corundum_, but that same material is referred to
in the Old Testament thus (in speaking of wisdom), "She is more precious
than _rubies_." It is obviously necessary to keep and to use all such
terms as have been for years established in usage, even though they do
not agree with the scientific method of naming the particular mineral.
It is, however, necessary that any name, thus retained, should be
correctly used, and that it should not be applied to more than one
material. Thus the term _ruby_ should be reserved exclusively for red
corundum, and not applied to other red minerals such as garnet, spinel,
etc., as is too of