262 



SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



is to the end of the crystal ; in combination with these three pyramids are the basal planes 

 and a nan'ow hexagonal prism. The frequent repetition or oscillation between pyramids of 

 this kind gives rise to horizontal striations on the faces, such as is shown inj^ In Fig. 53, i, 

 a combination of two hexagonal bipyramids with the hexagonal prism and an acute 

 rhombohedron is shown. As mentioned above, crystals of corundum occur in one of two 

 habits, that is with either the prism or the hexagonal pyramids predominating. The former 

 is the more characteristic for red corundum or ruby, and the latter for blue corundum 

 or sapphire. A crystal of ruby is shown in Plate I., Fig. 5, and a sapphire crystal in 

 Fig. 7 of the same plate, both stones being represented in their natural colours. 



J. h. 



Fig. 53. Crystalline forms of corundum {a — d, ruby ; c — i, sapphire). 



Twin intergrowths are often met with in cloudy corundum, but less frequently in 

 transparent material suitable for use as gems. These are of two kinds : in the one, large 

 numbers of thin plane lamellae, parallel to a face of the primitive I'hombohedron, occupy 

 twin positions in the crystal, as indicated in Fig. 53, a ; in the other, illustrated by h of the 

 same figure, the lamellae are parallel to the basal planes. Owing to this cause the faces of 

 crystals often bear fine striations, which differ in direction in the two kinds of twin 

 intergrowth. In the first, the striations are present on the basal plane in three sets of 

 parallel lines intersecting at 120° or 60°, and on the prism and rhombohedron faces in two 

 sets intersecting at an oblique angle. In the other kind of twin intergrowth, the basal 

 planes are smooth, while the faces of the prism and rhombohedron are horizontally striated^ 

 the striations on the prism faces being perpendicular to the prism edges. 



This twin lamination of corundum crystals makes the mineral appear to have two 

 cleavages, one parallel to one or more faces of the primitive rhombohedron and the other 

 parallel to the basal plane. Such a crystal when dropped or struck with a chisel will 



