CATALOGUE OF GEMS AND PRECIOUS STONES. 207 



Optical fluorite must yield or contain pieces at least one-fourth of an inch in diam- 

 eter, which must be clear and colorless and free from all defects. Defects consist of 

 internal cracks or cleavage planes, bubbles, or inclusions of dirt or mineral matter. 

 The presence of faintly developed or incipient cleavage planes or fracture surfaces 

 usually may be determined, if not readily visible, by moistening the specimen 

 with kerosene. The material must not show any anomalous double refraction. 

 Absolutely clear-water material is of the highest value, but very faint tints of green, 

 yellow, or purple do not render the material useless. 1 



Suitable material has been obtained from several of the fluorite mines in Hardin 

 County, Illinois, and may also occur in the extension of this fluorite belt in western 

 Kentucky. Although fluorite is found in many other States, practically none of 

 them is known to contain any "optical fluorite." 



Quartz. — See Rock crystal. 



Rock crystal. — The perfectly clear and colorless variety of quartz is called rock crystal. 

 It furnishes the material for certain special glasses and fused silica ware; and it is 

 used in wedges for microscopic work, as spectographic prisms for special reeeaiches 

 and as mechanical bearings. A use in connection with certain sounding boxes has 

 recently been developed. 



Sepiolite.—See Meerschaum. 



Sapphire. — The variety of gem corundum used for other purposes than jewelry is 

 called sapphire, irrespective of its color. It is used for mechanical bearings and 

 pivot supports, especially in watches and phonograph needles (mostly artificial 

 sapphire). 



Topaz. — Abrasive. 



Tourmaline. — In the tourmaline tongs or in polarizing forceps, a very simple form of 

 polariscope. 



Fluorite suitable for optical use is valued at from $1 to $10 a pound, according to the size of the piece 

 suitable for cutting as well as to its quality. The present yearly requirement is not large — perhaps sev- 

 eral hundred pounds— but under proper conditions and with a dependable steady supply this require- 

 ment may be increased. 



Possible, buyers of optical fluorite are: Bausch & Lomb Optical Co., Rochester, New York; Spencer 

 Lens Co., Buffalo, New York; Ward's Natural Science Establishment, Rochester, New York; United 

 States Bureau of Standards, Washington, District of Columbia. 



