CRYSTALS; CRYSTALLOGRAPHY; CLEAVAGE; and PHYSICAL CRYSTALLOGRAPHY. Other physical characters, however, which are not dependent or notably dependent on the direction the test is made, are, nevertheless, important and some of these may be described as follows: Luton, in the mineralogical sense, is not the degree of brilliancy, but the kind of brilliancy. Light reflected from different substances and quite independently of the color, produces dif ferent effects; one substance resembles a metal, another glass, another silk, and they are said to possess respectively metallic lustre, vitreous lustre and silky lustre. The determining causes appear to be transparency, structure and re fractive power. The most used terms are: Metallic lustre exhibited by those opaque min erals which with the exception of the native metals have a black or nearly black powder. Non-metallic lustre exhibited by all transparent or translucent minerals, which is subdivided into vitreous, adamantine, resinous, pearly, silky and waxy according to the similarity in sheen to glass, diamond, resin, mother of pearl, silk and wax respectively.
Color, by either transmitted or reflected light, depends upon the power of the sub stance to absorb different proportions of the lights of different wave lengths which together make up the light used. The same substance may, therefore, appear of different colors when viewed with different sources of light; and some minerals are strikingly different as, for nstance, alexandrite, which by daylight is bluish to olive green and by lamp or gas light raspberry red. Color is one of the least con stant mineral characters and varies with dif ferent specimens of the same species. The variation may be due to a few hundredths of 1 per cent of some organic or inorganic sub stance dissolved in the mineral, or to larger amounts of mechanically included foreign mate rial. Color effects may also be due to inter ference of light, usually as a result of some imperfection in the substance, or, in cut stones, from some purposely chosen shape producing notable dispersion of the white light into its component colors. Such effects are known as play of color, iridescence, opalescence, aster ism, etc.
Streak, is the color of the fine powder of the mineral and is nearly constant, no matter how the color of the mass varies.
Hardness, to the mineralogist, means the re sistance to abrasion of a smooth surface by a pointed fragment. It is usually, though very crudely, determined by comparison with the following scale introduced by Mohs: (1) Talc; (2) Selenite; (3) Calcite; (4) Fluorite; (5) Apatite; (6) Orthoclase; (7) Quartz,- (8) To paz; (9) Sapphire; (10) Diamond. Inter mediate values are window glass 5.5; jeweler's file 6.5 ; zircon 7.5; chrysoberyl 8.5; carborun dum 9.5. The more common procedure is to use pointed fragments of the scale minerals to scratch smooth surfaces of the mineral being tested. Sometimes this is more conveni ently reversed and roughly polished plates of the scale minerals are tried by sharp edges or points of the mineral. The members of the scale are not in arithmetical ratio. The aver age of five attempted comparisons from 9 down give, roughly, sapphire 100, topaz 30, quartz 18, orthoclase 12, apatite 7, fluorite 31/4, calcite 21/4, gypsum 1/4. The scale, nevertheless, serves a useful purpose and no convenient substitute has yet been suggested. Elaborate tests with a dia mond point, loss of weight by grinding with a standard powder, production of a crack by impact or pressure have been tried but fail to agree even approximately. .
The Specific Gravity of a mineral, as of any other chemical substance, is of first rank as a test and is a function of the density of the molecule. As explained in the article CHEMICAL CRYSTALLOGRAPHY comparative molecular vol umes are obtained by dividing the molecular weights by the specific gravities.
The specific gravity of a substance is de fined as its weight divided by the weight of an equal volume of distilled water at 4° C. The range, in varieties of the same species, is not great and even this is principally due to actual differences in composition. The value is usually obtained by means of a delicate balance provided with attachments for weighing the substance in water, such as a small wooden bench to hold a beaker of distilled water above the scale pan, and a platinum spiral to hold the specimen. Three weighings are needed: W= weight of the stone. S=weight of the spiral when suspended from the end of the balance frame and immersed in the distilled water. weight of the stone and the spiral suspended in distilled water.