512 REPORT OF NATIONAL MUSEUM, 1900. 



not used as a gem. 2. Satin spai\ a fine fibrous variety having the 

 pearly opalescence of moonstone and affording the "cat's-eye" raj' 

 when cut cabochon. Though soft, it is frequently worked up into 

 beads, pins, and other ornaments. 3. Alabaster^ a fine-grained, white 

 or delicately clouded variety. It is worked up into carvings, statuettes* 

 and other ornamental objects. 



Gypsum often forms extensive beds in connection with various 

 stratified rocks, especially limestones and marlytes; found also in crys- 

 talline rocks, about the fumaroles of volcanoes, and is deposited on the 

 evaporation of sea water and brines, in which it exists in solution. 



One hundred parts contain: Sulphuric acid, 46.51; lime, 32.56; water, 

 20.93. 



HEMATITE— CHROMIC IRON— ILMENITE. 



The compact, fibrous kinds of hematite are cut into beads, intaglios, 

 charms, and other ornaments. The color is iron black to steel gray; 

 luster metallic and opalescent; hardness 6; specific gravity -1.5 to 5.3. 

 One hundred parts contain: Iron, 70; oxygen, 30. 



Chromic iron and ilmenite have a use similar to that of hematite. 

 The hardness, specific gravity, and color of both are also near that 

 of hematite. Chromic iron contains: Iron, 38; chrome, 62. Ilme- 

 nite consists essentially of the oxides of iron and titanium in varying 

 proportions. 



HORNBLENDE. 



The use of hornblende as a gem is limited to those specimens found 

 penetrating liquid quartz. As such, especialh" when the hornblende 

 is in delicate acicular crystals, interlaced and penetrating the quartz in 

 ever}' direction, it affords very beautiful gems. 



lOLITE. 



DICHROTTE SAPHIR d'eAU — WATER SAPPHIRE. 



lolite, called also dichroite and saphir d'eau, or water sapphire, is 

 occasionall}' used as a gem. It occurs in orthorhombic crvstals of a 

 prismatic habit, and massive, compact. The hardness is greater than 

 that of quartz ; its specific gravity is 2. 60 to 2. QQ^ and its luster vitre- 

 ous. The colors are the various shades of blue, commonlj^ a smoky 

 blue, either light or dark. Its pleochroism is strongl}' marked, the 

 crystals presenting different shades of blue, bluish white, and yellow- 

 ish gray, according to the direction in which the}' are viewed; hence 

 the name dichroite, meaning two colored. 



lolite occurs in granite, gneiss, chloritic and talcose schists, and 

 allied rocks; also in igneous rocks, and as a contact mineral in connec- 

 tion with eruptive dikes, as in slates adjoining granite. The saphir 

 d'eau occurs in small rolled masses of a strong blue color in the alluvial 

 deposits of Ceylon. The mineral alters readily on exposure, so that it 



