PRECIOUS STONES. 93 



some practice with the various kinds of translucent or trans- 

 parent stones of various shades and colours has been acquired. 

 A preliminary examination of a few sapphires and rubies, 

 spinel rubies, garnets, topazes, tourmalines (green, brown, 

 red), zircons of various colours, andalusite, water sapphire, 

 coloured quartz (including amethyst), &c., will impart a 

 confidence very much more than any tabulated results of 

 dichroism, which depends much on the intensity or depth 

 of colour in the stone. 



Placing (by means of a tweezers) a translucent or trans- 



FIQ. 51. EXAMINING A GEM THROUGH THE DICHEOISCOPE. 



parent stone close to the one end of the instrument where 

 the two square images are seen when the instrument, held 

 skywards, is looked into, and turning it about in various 

 directions, and at the same time turning the instrument 

 round, the observer will notice whether the colour of the 

 two squares is one and the same. If the stone is amorphous, 

 such as glass, flint, obsidian, &c., or crystallizing according 

 to the cubic system, such as diamond,* spinel ruby, garnet, 

 &c., the two squares will be of the same colour. In other 

 cases the colour of one square will be of a different colour 

 to that of the other when the coloured stone is examined in 

 certain directions, though it may be the same in certain 

 others. 



Thus a true ruby, which affords two shades of pink, can 



* N.B. Colourless gems of any kind do not show two distinct 

 colours, and so the coloured diamond is here suggested. 



