COMPOUNDS OF ALUMINUM. ‘198 
black. The red shades often transparent and bright; the 
dark shades usually opaque. Lustre vitreons. H.=8. 
G.=3°5-4°1. | 
} Ali /N 
GS 

Composition. MgAl O0,=Mg 0+ Al0,= Alumina 72, mag- | 
nesia 28=100. ‘The aluminum is sometimes replaced in 
part by iron, and the magnesium often in part by iron, cal- 
cium, manganese and zine. Infusible ; insoluble in acids. 
Varieties. ‘lhe following varieties of this species have 
received distinct names: the scarlet or bright red crys- 
tals, spinel ruby ; the rose-red, balas-ruby ; the orange-red, 
rubicelle ; the violet, almandine-ruby ; the green, chloro- 
spinel ; while the black varieties are called pleonaste. Pleo- 
naste crystals contain sometimes 8 to 20 percent. of oxide 
of iron. Picotite 1s a variety containing 7 per cent. of 
chromium oxide. 
Diff. The form of the crystals and their hardness dis- 
tinguish the species. Garnet is fusible. Magnetite is at- 
tracted by the magnet. Zircon has a higher specific gravity 
and is not so hard. The red crystals often resemble the 
true ruby (red corundum), but the latter are never in octa- 
hedrons. 
Obs. Occurs in granular limestone; also in gneiss and 
voleanic rocks. At numerous places in the adjoining coun- 
ties of Sussex in New Jersey, and Orange county, of various 
