DESCRIITIVE CATALOGUE OF THE COLLECTIONS OF GEMS 

 IN THE UNrrED STATES NATIONAL MUSEUM. 



By Wirt Tassin, 

 Assistant Curator, Division of Mineralogy. 



I. DEFINITION AND PROPERTIES OF GEM MINERALS. 



A gem mineral may be defined as a mineral of any sort distin- 

 guished, especially when cut and polished, for its beauty, durability, 

 or rarity. 



The essentials, beaut}'^ and durability, are dependent upon the color, 

 brilliancy, and hardness of the stone, and these in turn are dependent 

 upon certain chemical and physical properties characteristic of any 

 one kind of mineral as compared with all other kinds of minerals. A 

 detailed description of all these properties is to be found in any text- 

 book of mineralogy, so that a discussion of them here will be limited 

 to those that will afford a more or less read}" means of distinguishing 

 one kind of gem from another or upon which their beauty largely 

 depends. These several properties are: 



Color. — The character of the color in ^\\\ one kind of gem is not a 

 constant, and may vary within rather wide limits. The garnet, for 

 example, which popularly is supposed to be a blood or purplish red 

 stone, varies through red of several shades to l)rown, black, green, 

 yellow, and nearly white. The color depends upon the power of 

 absorbing certain portions of light and reflecting others — that is, absorb- 

 ing certain rays of the spectrum that pass through or fall upon its 

 surface. Thus a gem appears red because it a))sorbs all other colors 

 and reflects, or transmits chiefly the reds; a gem appears green because 

 it reflects, or transmits chiefl}^ green rays, absorbing all others. In 

 all cases the color results from the constituents of the light which 

 have not been absorbed. Those which reflect or transmit all the colors 

 in the proportions in which they exist in the spectrum and absorb 

 none are white; those which absorb all and reflect or transmit none 

 are black. Between these two limits there is an infinite variety of 

 hues, according to the greater or less extent to which substances reflect 

 or transmit some colors and absorb others. 



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