CORUNDUM: PHYSICAL CHARACTERS 



separate along plane surfaces in these directions. This splitting up, which is known as a 

 platy separation, is not a true cleavage but is due to want of cohesion between the individual 

 lamellae. If the division were due to cleavage the lamellas after separation could be split 

 up again in the same direction, which is not the case. Moreover, the property of splitting 

 along surfaces parallel to the faces of the rhombohedron and to the basal plane is possessed 

 only by those crystals which are penetrated by twin-lamellae parallel to these faces. In 

 crystals in which twin-lamellae are absent all trace of platy separation is also absent, the 

 fracture being conchoidal as is the ease with many other non-cleavable minerals. 



Corundum is brittle, and among other physical characters it possesses that of hardness 

 to a high degree. After diamond, it is the hardest of all minerals, having the number 9 

 assigned to it in Mohs' scale of hardness. It is, however, much more nearly approached in 

 hardness by certain other minerals than it itself approaches diamond in this respect. Its 

 hardness renders this mineral very valuable as a grinding and polishing material, the cloudy 

 and opaque varieties, especially the compact black emery, being much used for this purpose, 

 v/hile rather superior qualities are utilised for the pivot supports of watches and various 

 delicate instruments. There are degrees of hardness among the different varieties of 

 corundum, the blue sapphire standing first in this respect. 



The specific gravity of corundum is very high ; that of pure material is very near to 4, 

 values varying between 3-94 and 4<'08 having from time to time been determined on pure 

 material. Greater variations on either side of the mean value of 4'0 are due either to errors 

 of observation or to the determinations having been made on impure material. No difference 

 has been observed in the specific gravity of differently coloured varieties. Corundum is 

 thus one of the densest of precious stones, and this character renders it easily distinguishable 

 from other minerals which may resemble it in general appearance. It sinks rapidly, not 

 only in methylene iodide but also in the heavier liquid (sp. gr. =3"6) obtained by dissolving 

 iodoform and iodine in methylene iodide. 



It is not attacked by acids either in the cold or when warmed, and is completely 

 infusible before the blowpipe. Many specimens of corundum when heated in a 

 dark room display a beautiful phosphorescence. When rubbed with cloth or leather 

 the mineral acquires a charge of positive electricity, which it retains for a considerable 

 time. 



Corundum varies greatly in appearance. Most frequently it is cloudy and opaque, and 

 only a small proportion is clear and transparent and valuable from an aesthetic point of 

 view. The common cloudy and opaque varieties of corundum will not be here considered in 

 detail, but our attention will be directed to the transparent varieties known as noble or 

 precious corundum. 



The lustre of precious corundum is very fine, and is displayed to great advantage by 

 the polished facets of a cut stone. It approaches that of diamond in brilliancy, but differs 

 from it in character, being not adamantine but vitreous, like that of many other precious 

 stones and glass. So brilliant and perfect a lustre and so marked a fire is displayed by no 

 other precious stone with the exception of diamond, and perhaps also of colourless hyacinth 

 (zircon), so that by these characters alone the stone may be distinguished from other gems 

 of the same colour. Even when a specimen of corundum is of a poor colour it will still be 

 cut as a gem, since the fine 1 ustre of the mineral will redeem the stone from insignificance. 

 Moreover, owing to the great hardness of this stone, the lustre is retained even after rough 

 usage and hard wear. 



Corimdum is optically uniaxial and doubly refracting. Its refraction, though strong, is 

 considerably less than that of diamond ; its double refraction is small, the refractive indices 



