Precious Gem stones production in 1918 Page 72



	GEMS AND PRECIOUS STONES.	13

	incipient cracks along fracture lines, nor any cleavage, nor twinning planes. Neither can there be any capillary or larger tubelike cavities, nor cavities or bubbles of any shape, nor inclusions, as isolated particles, veins, or clouds, composed of minute crystals of some other mineral or of any kind of foreign substance. The spar should not be discolored or stained by the presence of any clay, iron oxide, or other material. It should be noted that many of the inclusions and imperfections of Iceland spar are not always scattered irregularly through the mineral or even segregated in distinct masses, but frequently lie in a distinct but very thin plane which can hardly be seen if looked at on edge. In examining a piece of Iceland spar for defects the piece should therefore be turned in all directions while held to the light. The material suitable for optical uses naturally brings the highest prices, as it has to be at least of the dimensions already given. Specimen material isgenerally of a larger size. The material used for standardization, chiefly chemical, need be of no special size, and the smaller pieces are as usable as the larger ones. The optical variety of Iceland spar produced in the United States, sold, per pound, for $3 to $4in 1914, about $8 in 1915, and as high as $20 in July, 1918. The specimen variety sells for considerably less, and material for standardization sells for from $1 to $2 a pound. The following firms are buyers of Iceland spar suitable for optical use: Bausch & Lomb Optical Co., Purchasing Department, Bochester, N. Y.; Central Scientific Co., 460 Ohio Street east, Chicago, 111.: Gilbert S. Dey, Superintendent Optical Department, Eastman Kodak Co., Bochester, N. Y. The market for specimen spar is irregular, as the demand is usually very light. The best market will probably be found with some of the larger mineral dealers. Standardization material may be sold to large dealers in general chemicals as well as to mineral dealers. Although calcite is, next to quartz, the commonest mineral, the only locality out-sideof Iceland known to produce the variety Iceland spar in commercial quantity is in Montana, about 9 miles from Gray Cliff, Sweet Grass County, on the main line of the Northern Pacific Bailway The spar occurs in a nearly vertical fissure vein from 3 to 8 feet thick, which strikes northwest, traversing a gneissic rock for several miles. Brief mention of the Montana occurrence of Iceland spar is made in the reports on the production of gems and precious stones in Mineral Resources for 1913 (p. 704) and 1914 (p. 335). C. L. Parsons, of the Bureau of Mines, has also described the occurrence and material in Science, vol. 47, No. 1221, pp. 508-509, May 24, 1918. Jasper. See Agate. Malachite. Ore of copper, pigment for paint. Mariposite. Pigment for paint. Meerschaum. Pipe bowls; cigar and cigarette holders. Optical fluorite. Fluorite, commonly called fluorspar, is a common mineral but is very seldom found in pieces clear enough and large enough to be of special use in the manufacture of certain optical lenses and prisms. Fluorite of the requisite quali- ties, as described below, suitable for such use is known as "optical fluorite." Any deposit of fluorite may yield a small quantity of such material, but at present about the only localities known to produce it are southern Illinois; Meiringen, Switzerland; and Obira, Bungo, Japan. Optical fluorite is cut into lenses and placed between glass lenses. It forms the apochromatic objective for micro- scopes and similar optical instruments, the fluorite lens correcting the spherical and chromatic errors of the glass lens systems. This result is due to the low refrac- tive power, weak color dispersion, and single refraction of fluorite. These apo- chromatic lenses represent the finest type of microscope objectives made. The use of such a fluorite lens greatly increases the value of a microscope and if optical fluorite were more abundant many more microscope objectives would be equipped with such lenses. Optical fluorite is also used in the lenses of certain telescopes, in making prisms for spectrographs in ultra-violet work, and in other optical apparatus where transparency in the ultra-violet and infrared parts of the spectrum is necessary. Optical fluorite must yield or contain pieces at least one-fourth of an inch in diame- ter, which must be clear and colorless and free from all defects. Defects consist of internal cracks or cleavage planes, bubbles, or inclusions of dirt or mineral matter. The presence of faintly developed or incipient cleavage planes or fracture sur- faces usually may be determined, if not readily visible, by moistening the speci- men with kerosene. The material must not show any anomalous double refrac- tion. Absolutely water-clear material is of the highest value, but very faint tints of green, yellow, or purple do not render the material useless.