CORUNDUM 261 



CORUNDUM. 



Some of the most beautiful and valuable of precious stones, including the red ruby 

 and the blue sapphire, belong to the mineral species corundum. All such stones are 

 alike in the possession of those physical characters which essentially define a mineral 

 species. Their appearance, however, owing to the great variety of colour displayed, 

 »nay be very diverse, and thus the species furnishes a large number of gems, each with a 

 ■distinct and characheristic colour, but having the same chemical composition and crystalline 

 form. 



Chemically considered, corundum is pure alumina, the oxide of the now much used 

 metal aluminium. The. chemical formula AlgOg, by which this oxide is represented, 

 corresponds to 53'2 per cent, of metal and 4<6"8 per cent, of oxygen. Chemical analyses 

 of naturally occurring corundum, however, always show the presence of some impurity, the 

 amount of which is smaller the clearer and more transparent is the material used in the 

 analysis. Foreign impurities are sometimes present in large amount, up to 10 per cent, or 

 even more, and when this is the case the stone is rendered cloudy and loses its assthetic 

 value. The analysis of natural corundum has demonstrated the presence of iron oxide, 

 silica, and occasionally traces of chromium oxide. The chemical composition of a beautiful 

 transparent red corundum, the so-called " oriental ruby,'" and of a blue corundum or 

 " oriental sapphire " of equally fine quality, is given below : 



99-62 100-20 



It is on the presence of such foreign substances as iron oxide and in part probably on 

 chromium oxide that the variety of colour found in this species depends. 



Corundum occurs not infrequently in well-developed crystals belonging to the 

 rhombohedral division of the hexagonal system. A series of the more frequently occurring 

 forms, which are of two different habits, is shown in Fig. 53, a — i. In some a hexagonal 

 prism is more or less largely developed and terminated at the two ends by basal planes 

 perpendicular to the prism planes, faces of the primitive rhombohedron occupying alternate 

 comers. Most of the crystal-faces are smooth ; the basal planes, however, bear regular, 

 triangular striations, as in the crystal shown in Plate I., Fig. 5. Crystals with these three 

 forms are shown in Figs. 53, a, b, c ; in a and b the prism predominates, the only difference 

 between the two being the greater size of the rhombohedron faces in 6 ,- in c the prism faces 

 are narrow, while the rhombohedron faces and the basal planes are all about equal in size. 

 Fig. 53 d shows a combination of the same forms with a double hexagonal pyramid in 

 addition, the twelve faces of which replace the edges between the prism faces and the basal 

 planes. This hexagonal bipyramid is present in all the remaining figures, e to i. In Fig. 

 53 e it occurs alone, and in the remaining forms it is predominant ; in y it is in combination 

 with the basal planes, and in g with the basal plane and a rhombohedron, the faces of which 

 replace alternate corners above and below. In h there are three such hexagonal bipyramids 

 with different inclinations, each successive pyramid being less steeply inclined the nearer it 



