EARTHY MINERALS. 63 



mond. It is the sapphire, and is of various colors. It 

 is only the blue crystals that commonly receive this 

 name, while a red crystal of it is called the Oriental ruby, 

 and a yellow one the Oriental topaz. This mineral is in 

 hardness inferior only to the diamond, scratching quartz 

 quite easily. Alumina is the essential ingredient of all 

 clay, and is therefore very abundant, entering into the 

 composition of many rocks, and forming a constituent 

 more or less of the soil. It is calculated that aluminum, 

 its base, is one twentieth part of the crust of the earth. 



127. Spinel. This mineral is composed essentially of 

 alumina and magnesia intimately combined, there being 

 small quantities in it of other substances, as oxyd of 

 iron, silica, and chromic acid. The finely-colored crys- 

 tals are prized as gems in jewelry, and the red is the 

 common niby, in distinction from the Oriental, which is 

 a sapphire. There are some varieties of spinel in which 

 the oxyd of zinc is a prominent ingredient. 



128. Silicates of Alumina. There are a few silicates 

 of alumina that have no other substance in combination, 

 one of which is named Sillimanite, in honor of Professor 

 Silliman, sen., of Yale College. But most of the silicates 

 of this earth are quite compound, having various oxyds 

 and other substances combined with them, as lime, 

 magnesia, potash, soda, oxyd of iron, lithia, oxyd of man- 

 ganese, etc. I will notice a few of those which are most 

 prominent in interest. 



129. Feldspar. This mineral, which is a silicate of 

 alumina and potash, is one of the three crystalline con- 

 stituents of granite, and its partially formed crystals are 

 easily distinguished in the coarser specimens of that 

 rock. It is not as hard as quartz, and is more brittle. 

 Its crystals are commonly four or six-sided prisms, and 

 have a pearly lustre. Two of its usual crystals are rep- 

 resented in Fig. 23 (p. 64). The 'colors of this mineral 

 are commonly white, milk-white, gray, and flesh-colored, 

 but sometimes the crystals are violet and green. Some- 



