CATALOGUE OF GEMS. 487 



F(n'm. — The external form of a gem mineral may be described as: 

 A — crystal lized. B — crystalline, and ^ '^amorphous. 



A — Crystallized: ^Vhen made up of geometrical solids, any indi- 

 vidual polyhedral form of which is called a crystal. Crystals are 

 bounded hy plain surfaces called planes or faces, symmetricallj'^ 

 arranged with reference to one or more imaginar}" ' diametral lines 

 called axes. 



Crystals, though their forms and modifications are of an idefinite 

 number, are classified under six systems according to the num])er and 

 character of their axes. The systems of crystallization and tj^pical 

 examples occurring under each are: 



Isometric. 



Examples: Cube, octahedron, and dodecahedron. 

 Tetragonal. 



Examples: Square prism and square octahedron. 

 Hexagonal. 



Examples: Hexagonal prisms, pyramids, and rhombohedrons. 

 Ortliorhoml)ic. 



Examples: Right prism on a rhombic base and a rhombic; octahedron. 

 Monoclinic. 



Examples: Oblique prisms on a rectangular base and oblique octahedron on 

 a rhombic base. 

 Triclinic. 



PLxamples: Doubly oljlique i)rism and doubly oblique octahedron or pyramid. 



B — Crystalline: When the mass appears to be made up of closely 

 compacted, minute crystals. The individuals composing crystalline 

 masses may be: In columns or fibers, in which ease the structure is 

 columnar: in thin laminae, plates, or leaves, giving rise to a lamellar 

 structure; in grains, producing a granular structure. Further, there 

 are numerous irregular and accidental groupings of the individuals 

 composing the mass, giving to it certain shapes, such as globular, 

 botryoidal, reniform, dendritic, etc., which are too numerous to allow 

 of a specification here. These indeterminate forms are grouped luider 

 the head of "imitative shapes." 



( ' — Amorphous: Finally, the mass ma}^ be entirely destitute of crys- 

 talline structure or imitative shape and show neither external nor 

 internal signs of crystallization. Such a mass is said to be amorphous. 



II. DESCRIPTION OF MINERALS USED AS GEMS. 



ALBITE. 



AVENTURINE — MOONSTONE — PERISTERITE. 



Albite occurs in opaque to transparent masses and in triclinic crystals 

 having a cleavage in two directions, one of which is highl}' perfect. 

 The mineral has a hardness of 6; a specific gravity of 2.62, and a 

 vitreous luster, often pearly on a cleavage surface. Color white, also 



NAT MUS IHdO 34 



