Precious Stones: Spinel - Chrysoberyl Page 204



	PRECIOUS STONES.	199

	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