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tarnish. Non-metallic minerals are transparent on thin edges or 

 in thin sections, such as rock sections, which are under .04 mm. 

 M.uiv minerals which are opaque in coarse fragments, as the black 

 tourmalines or rutile, will be transparent in thin sections; even 

 met.tk as gold, will transmit light if the sheets are thin enough. 

 The opacity to light is relative, depending upon the thickness of 

 the section. A mineral is said to be transparent if the outline of 

 olijects can be distinguished through it; translucent when the 

 light is diffused and the outlines of objects are no longer distin- 

 guishable through the specimen. Some effects are due to both 

 diffusion and interference of light, as opalescence, and the milki- 

 ness of some quartz, the play of colors in the fire opal, or the color 

 yielded by some labradorite when viewed at certain angles with 

 the twinning planes. The star sapphires and cat's-eye are due to 

 reflections and a fibrous structure. The prism colors or banded 

 effects produced by films of air in thin cleavage cracks are due to 

 interference. 



The variability of color in non-metallic minerals is well illus- 

 trated in quartz, a mineral which, when pure, is colorless. Very 

 small amounts of some oxides which are present as impurities act 

 as a pigment, yielding decided colors even when the amount of 

 coloring material is so small as not to be detected by the ordinary 

 chemical tests. 



Certain colors are characteristic of chemical elements, as most 

 copper minerals are blue, green, or red; and copper compounds will 

 yield these colors when present in other minerals as impurities or 

 inclusions. Calcite, smithsonite, and quartz are often colored 

 green by copper compounds. Chromium yields a green color, as 

 in the green of some garnets and in the emerald. The red of the 

 ruby as well as the red of crocoite is chromium in a different form 

 or ion. Nickel also yields a green color, as in chrysoprase, a variety 

 of quartz. Iron as an impurity will yield shades of red, brown, 

 yellow, green, or blue according to its state of oxidation or combi- 

 nation. The yellow calcites of Joplin, Missouri, contain ferrous 

 iron carbonate and the red jaspers and bloodstones are colored by 

 ferric iron. Very small quantities of manganese yield intense colors; 

 pinks, amethystine, and some greens are caused by manganese. 

 The rose color of some quartz and tourmaline is caused by titanium, 

 which also when in a lower state of oxidation will yield a blue. 

 Cobalt is the most intense of all mineral pigments and many blue 

 minerals owe their color to cobalt; when in cobaltic form, the ion is 



