CATALOGUE OF GEMS AND PRECIOUS STONES. 7 



ceding in the series and be scratched by all following. In general 

 only those minerals which possess a hardness greater than 6 in this 

 scale — that is, which will at least scratch orthoclase — are sufficiently 

 durable to be used as precious stones. 



Specific gravity. — The specific gravity of a substance is its weight 

 compared with an equal bulk of pure water; thus, the statement 

 that diamond has a specific gravity of 3.5 means that it is three and 

 one-half times as heavy as water. This property is characteristic of 

 many precious stones and is of considerable value in their identifi- 

 cation. It is determined by weighing the stone first in air and 

 then suspended in water. The first weight, divided by the difference 

 between the two, gives the value desired. For details as to the 

 apparatus used for this purpose, books on mineralogy or physics 

 must be consulted. 



Optical properties. Index of refraction. — -When light passes ob- 

 liquely from one transparent substance to another, its direction is, 

 in general, altered at their boundary. The well-known phenomenon 

 of the apparent bending of a stick thrust into water is an illustration 

 of this. The extent to which light entering minerals from the air is 

 shifted is called their index of refraction; in mathematical terms this 

 is the ratio between the sine of the angle of incidence and that of the 

 angle ol refraction. In amorphous minerals and in those crystal- 

 lizing in the isometric system, the index of refraction is the same in 

 all directions; in those crystallizing in the systems derived from a 

 revolution ellipsoid — namely, the hexagonal, trigonal, and tetrago- 

 nal — there are two different indices; and in those crystallizing in the 

 remaining systems there are three. The last two groups are said to 

 possess double refraction, which in the first is the difference between 

 the two indices; in the second, the difference between the largest 

 and smallest. 



In a few minerals, notably in Iceland spar (transparent calcite), 

 the double refraction is so strong that an object observed through 

 the mineral appears double; in most cases, however, the double 

 refraction is too slight to be rendered visible in this manner, and 

 special means must be employed for its recognition; polarized light 

 (light the vibration of which is limited to a definite plane) is generally 

 used for this purpose. For details of the phenomena connected with 

 polarized light, books on optical mineralogy must be consulted. It 

 may be mentioned here, however, that it is most conveniently ob- 

 tained by causing ordinary light to traverse prisms of Iceland spar so 

 constructed that only one of the two rays into which the light is split 

 is actually transmitted. The light emerging from such a prism 

 (called a "nicol" after its inventor) is vibrating in a single plane; 

 and if two nicols are disposed so that the planes are at right angles 

 no light will be able to traverse the svstem. A substance with single 



