CATALOGUE OF GEMS AXD PRECIOUS STONES. 5 



patterns, each repeated twice, in opposite directions, all again at 

 right angles; the crystals of this system are made up of combina- 

 tions of pinacoids, prisms, domes, and pyramids. 5, Monoclinic, 

 system, comprising crystals developed like the orthorhombic, but 

 with the patterns reappearing in one plane twice at angles less, and 

 twice at angles greater, than right angles; the forms are like those 

 of the preceding system. 6, Triclinic system, comprising crystals 

 developed like the two preceding systems, but with none of the 

 patterns repeated exactly at right angles to one another; the forms 

 are essentially the same as those of the two preceding systems. 



In addition, a few minerals have their atoms irregularly arranged, 

 and consequently are without crystal form; to these the term amor- 

 phous is applied. 



Color. — The color of an object expresses the character of the ligh 

 reflected or transmitted by it. The principal terms used in describ- 

 ing it are: White, gray, black, violet, blue, green, yellow, red, and 

 brown. There are also many intermediate hues, best described by a 

 combination of terms, as blue-green, etc. Colors may be divided 

 into two classes: 



1. Essential or idiochromatic colors, due to the chemical elements 

 which make up a compound. They are definite and characteristic 

 for each substance and only disappear when it is decomposed. The 

 principal elements yielding colors in the order of their most frequent 

 occurence in precious stones are: 



2. Nonessential or allochromatic colors, due to impurities or foreign 

 constituents in a substance. They may vary widely from one speci- 

 men to another and may be of three different types: 1, Produced by 

 evident, distinct particles, which in themselves show essential colors. 

 Thus, finely divided ferric oxides are frequently inclosed in other 



4555—22 2 



