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250 A Book of Precious Stones
similar physical or crystallographic features are arranged in smaller subdivisions. To quote Professor Gratacap:
The
forms of minerals are their most obvious characteristic. The six-sided
prisms of quartz and beryl crystals, the rhomboidal or trapezoidal
faces of garnet, the triangular faces of magnetite and the square faces
of fluorite are unmistakable.
The
branch of mineral science known as crystallography is now well
developed and established, and it has been demonstrated that crystal
form has a close dependence upon chemical composition. The arrangement
of all specimens at the American Museum of Natural History, in both
desk and wall cases, is exemp-larily systematic, and in accordance with
the classification of the sixth edition of Dana's System of Mineralogy. An
intelligent inspection of the collection at this museum, for the
novice in mineralogy, should begin with desk case No. 28, followed by
case No. 27; these two cases contain introductory series presenting the
chemical and physical features of minerals, together with explanatory
tables and photographs. The models showing the formation of crystals
are ingenious in design and excellent in construction, and illustrate
the crystallographic system to the
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