electricity, but not to any great degree. It is not fusible before the ordinary blow-pipe.
The specific gravity is 3-60 to -3-63,
rather higher than the Diamond. The fracture is conchoidal, and there
is a very imperfect octahedral cleavage. It is a brittle mineral,
though its hardness is considerable, being equal to 8 of Mohs' scale.
The streak is white.
The
crystalline form is cubic, and the usual habit octahedral (Fig. 15) ;
very rarely is the habit cubic ; the faces of the octahedron are
sometimes curved ; twinning of the crystals is commonly seen. Crystals
are usually idiomorphic and completely developed. An intimate lamellar
structure is sometimes seen, due to repeated twinning in one plane.
In
origin it is very closely allied with Corundum, being usually found in
limestones altered by thermo- or dynamo-metamorphism, thus naturally
its associates are the same as Corundum usually has.
In chemical composition it is a combination of the oxides of magnesium and aluminium, MgO, A1203.
The colour is due in all cases to the presence of other substances ;
thus traces of chromium or iron may cause a red colour ; these metals
are not so much impurities as replacement products, for some of the
aluminium may be replaced by ferric iron or chromium, and some of the
magnesium, by the iron (in a ferrous condition) or by manganese. We
thus get three other mineralogical varieties. Ruby Spinel or Magnesium
Spinel being the first, the second is an Iron-Magnesium Spinel, and is
known as Ceylonite or Pleonaste ; it is usually
