. 201. OPTICAL PROPERTIES- OF MINERALS. 285 



3. Yellowish-white. White (though not always of the 

 purest) inclining to yellow. Ex. Several varieties of rhom- 

 bohedral Lime-haloide and of uncleavable Quartz. 



4. Greyish-white. White, inclining to grey. Common 

 in rhombohedral Lime-haloide, particularly in the com- 

 pound varieties called granular Limestone, and in rhombo- 

 hedral Quartz, particularly in Common Quartz. 



5. Greenish-white. White, somewhat inclining to green. 

 Very distinct in several varieties of hemi-prismatic Augite- 

 spar, particularly in Amiantus, and in the varieties of pris- 

 matic Talc-mica, called Common Talc. 



6. Milk-white. White somewhat inclining to blue, the 

 colour of skimmed milk. Ex. Several varieties of uncleav- 

 able Quartz, called Common Opal. 



B. Grey. 



1. Blucish-grcy. Grey inclining to a dirty blue colour. 

 Seldom distinct. Sometimes in the varieties of rhombohe- 

 dral Quartz, called splintery Hornstone, and in several 

 compound varieties of rhombohedral Lime-haloide. 



2. Pearl-grey. Grey, mixed with red and blue. In the 

 pearls this colour is very pale. Sometimes it is very dis- 

 tinct in hexahedral Pearl-kerate ; less distinct in several 

 varieties of rhombohedral Quartz, and of prismatic Hal- 

 baryte. 



3. Smoke-grey. Grey mixed with brown ; the colour of 

 thick smoke. This colour occurs particularly in the dark 

 varieties of Flint, which belong to the species of rhombo- 

 hedral Quartz. 



4. Greenish-grey. Grey mixed with green. Ex. Seve- 

 ral varieties of rhombohedral Quartz, particularly Cats-eye ; 

 several varieties of rhombohedral Talc-mica, &c. 



5. Yellowish-grey. Grey mixed with yellow. This co- 

 lour is not uncommon in several compound varieties of 

 rhombohedral Lime-haloide (compact Limestone) and of 

 rhombohedral Quartz (Flint). 



6. Ash-grey. The purest grey colour, a mixture of 

 white and black. It is seldom distinct. E.v. Prismatoidal 



