Identifier: popularguidetomi00grat (find matches)
Title: A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy
Year: 1912 (1910s)
Authors: Gratacap, L. P. (Louis Pope), 1851-1917
Subjects: American Museum of Natural History. Bement Collection of Minerals Mineralogy
Publisher: New York : D. Van Nostrand
Contributing Library: Robarts - University of Toronto
Digitizing Sponsor: University of Toronto

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s the red and blue shades of color;the latter tint, if the form is pyramidal, being the deepest at thebase, and evincing a tendency to traverse the center of the crystal,nearly to its apex, where the ruby color wholly replaces it, andsometimes here presents itself with much intensity. The facesdififer considerably in smoothness and luster. Those belonging tothe prism, the primary rhombohedron, and the face perpendicularto the axis being the most perfect, while those of the pyramid arethe most deficient in finish. The double pyramids from Ceylonor N. Car. are irregular, indented, and roughened. The Montanasapphires are hexagonal prisms with basal plane, of a beautiful softblue, and extracted from a trap (igneous) rock. As a gem stonecorundum exhibits a surprising range of color, blue, red, amethys-tine, yellow, salmon tinted, white, greenish, with intermediateshades, and bronzy reflections. The star-sapphires so much valuedresult from tubular cavities arranged in planes parallel to the
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ARAGONITE (pseudomorphous) Cianciana, Sicily, ItalyBement Collection, American Museum of Natural History GUIDE TO COLLECTIONS 143 prism (Miers). It is dichroic, in ruby the colors are red and vio-let-red, in sapphire deep-blue and greenish-blue. The surface of corundum is often altered into hydrated pro-ducts, as silvery scales of margarite and damourite, and it is muchfound with aluminum hydrates, as bauxite, diaspore, gibbsite. Hematite, the sesquioxide of iron, is one of the most com-mon iron ores and disseminated through almost all soils and earths,contributes color to the landscape, and forms in rocks a pervasivecoloring substance also. Its crystals (rhombohedral) are oftenof great beauty, with lustrous or iridescent surfaces, and its spec-ular (mirrorlike) micaceous forms are brilliant features in thecabinet. The beautiful specimens from the Island of Elba, Italy,with their striated faces, blue and iridescent tarnish, and the ex-quisite rosettes or nests of embracing crystals fr

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