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 often done. It will be the purpose of this lesson to attempt to set forth as clearly and as briefly as possible what constitutes good usage in the naming of the principal stones, and also to point out what incorrect usage is most in need of being avoided. To cover the subject systematically we will adopt the order of hardness that we did in discussing mineral species in Lesson XVIII.