PHYSICAL I'HOl'KRTIKS 



277 





392, at right angles to the vertical axis of a tourmaline from 

 Bra/il. In the center there is an area of concentric hands of differ- 

 ent shades of pink, then a colorless band, outside of which is a band 

 of green. Again the colors may be distributed lengthwise the 

 ( ry-t.il, aa many specimens from Mesa Grande, California, in which 

 one end may be green, the other pink, and often separated by a 

 colorless middle band across the body of the crystal. 



The delicate coloring of some transparent crystals is often 

 changed by heating ; in this way the pink topazes are produced 

 by heating the originally yellow stones from Brazil. This process 

 is known as "pinking," and if the heat is too great, the crystal 

 becomes colorless. 



In the description of colors in minerals Werner, nearly two hun- 

 dred years ago, fixed the following eight colors as primary, and since 

 that time mineralogists have been accustomed to describe minerals 

 in terms of these eight colors with their variations as follows : 



WHITE, either clear, transparent, or translucent 



Snow-white, as Carrara marble. 

 Reddish white, as some calcite. 

 Yellowish white, as some calcite. 

 Grayish white, as some calcite. 



Greenish white, as talc. 

 Bluish white, as some calcite. 

 Milk-white, as some quartz. 



Blue-gray, as some wernerite. 

 Pearl-gray, as some dolomites. 

 Smoke-gray, as flint. 

 Greenish gray, as tremolite. 



GRAY 



Yellowish gray, as some dolo- 

 mites. 

 Ash-gray, as leucite. 



BLACK 



Grayish black, as ilvaite. Brownish black, as allanite. 



Velvet-black, as black tourma- Reddish black, as acmite. 



line. Bluish black, as tourmaline (in- 

 Greenish black, as augite. dicolite). 



BLUE 



Blackish blue, as dark azurite. 

 Azure-blue, as light azurite. 

 Violet-blue, as some fluorites. 



Prussian blue, as cyanite. 

 Smalt-blue, as dumortierite. 

 Indigo-blue, as some vivianites. 



Lavender-blue, as some sodalites. Sky-blue, as turquoise. 



