EARTHY 3IINERALS. 65 



same size in our own country in several localities in 

 New Hampshire. 



131. Garnet. In this mineral there are combined the 

 silicates of alumina, lime, iron, and manganese, and the 

 various colors of the different specimens generally come 

 from the difference in proportions of these ingredients. 

 The most common color is deep red. In an emerald- 

 green variety the color is caused by oxyd of the metal 

 chromium. Clear garnets of a deep red color are highly 

 prized in jewelry. 



132. Tourmaline. The usual form of the crystals of 

 this mineral is a prism terminating in a low pyramid. 

 They are commonly long, and the sides are apt to be fur- 

 rowed. The most common colors are black, blue-black, 

 and dark brown ; but, besides these, there are wide vari- 

 eties, as bright and pale red, grass-green, yellow, etc., 

 even to white. Though alumina is the chief base in all 

 the varieties, the differences in the composition of some 

 of the varieties are considerable. For example, in a black 

 variety there was found to be 24 per cent, of oxyd of iron, 

 while in a red variety there was no iron at all, but 5 per 

 cent, of the peroxyd of manganese, a frequent cause of 

 this color in minerals. Some red and yellow tourmalins 

 are of great value as gems. A Siberian tourmaline of 

 the red variety, now in the British Museum, is considered 

 to be worth 500. 



133. Topaz. This is said to be a silicate combined 

 with a fluorid. If so, it must be a combination of sili- 

 cate of alumina with fluorid of the metal aluminum, for 

 there can be no such thing as a fluorid of alumina any 

 more than there can be a chlorid of soda. In explana- 

 tion of this I refer you to 352, Part II. The clear crys- 

 tals of topaz are used in jewelry, and it is a curious fact 

 that the color can be altered for this purpose by exposure 

 to heat. The Brazilian topaz can be thus made so to re- 

 semble the real rose-red ruby that it can not be distin- 

 guished from it except by an electrical test. When the 



