122 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



gradations have been found. Such a cube of diamond in its matrix is shown in Plate I., 

 Fig. 1. 



In most cubes of diamond, however, each edge is replaced by two faces, as shown in 

 Fig. 31 a ; the twenty-four faces thus derived, would, if produced or enlarged sufficiently, 

 give rise to the form known as the four-faced cube, or tetrakis-hexahedron. These faces 

 are, however, as a rule, small ; they are dull and uneven, and are irregularly striated per- 

 pendicularly to the cubic edges. Each face is often divided centrally by a narrow furrow, 

 running perpendicularly to the edge and towards the centre of the cube face ; this is 

 illustrated in Fig. 31 b, which shows one cubic face together with the four adjacent faces of 

 the four-faced cube. A crystal, bounded only by the twenty-four faces of the four-faced 

 cube and with no cube faces, is occasionally met with in diamonds from Brazil and India ; 

 the faces of this form are then bright, but always curved. 



The cube is also frequently modified in ways other than by the replacement of its 

 edges. Not infrequently, for example, its eight corners are truncated by the eight faces of 

 the octahedron. Moreover, each of the twelve edges of the cube may be replaced by a 

 single plane face ; these twelve truncating faces, if extended, would give the form known as 

 the rhombic dodecahedron, which is of frequent occurrence, and is shown in Fig. 31 c and i. 

 Its faces are sometimes plane and striated parallel to the longer diagonal (Fig. 31 i) ; as a 

 rule, however, they are more or less curved, the lines of intersection of the faces being of 

 course also curved ; in this latter case the faces are not striated but are smooth and bright 

 (Fig. 31 c). These curved faces frequently have a shallow groove running across them in 

 the direction of the shorter diagonal, as indicated by the dotted line in Fig. 31 c ,- the form 

 is then, strictly speaking, no longer a rhombic dodecahedron, but approaches to that of a 

 tetrakis-hexahedron. The largest Brazilian diamond yet found, and known as the " Star 

 of the South," is an irregularly developed rhombic dodecahedron ; it is shown in its rough 

 condition in Fig. 48. This form is frequently to be met with in Brazilian diamonds. 



When the faces of the rhombic dodecahedron are grooved in the direction of the 

 longer diagonal as well as in that of the shorter (Fig. 31 d), we obtain a form known 

 as a hexakis-octahedron, bounded by forty-eight similar faces, which are always strongly 

 curved, smooth, and bright. The hexakis-octahedron, which is of extremely frequent 

 occurrence in diamond, approaches, as shown in Fig. 31 d, the form of the rhombic 

 dodecahedron ; at other times the same kind of form may approximate to the octahedron, 

 each of the eight octahedral faces being replaced by six faces. Hexakis-octahedral crystals 

 of the diamond are frequently much distorted by elongation in one direction, as shown in 

 Fig. 31 e, a still greater distortion of the same form being represented in Fig. 31 y. Such 

 distorted forms, which appear at a first glance to be quite distinct from that of Fig. 31 d, 

 on a closer examination will be seen to be easily derived from that form. 



Both the rhombic dodecahedron and the hexakis-octahedron are sometimes, on account 

 of the strong curvature of their faces, almost spherical in shape. Formerly, when the 

 principal localities for diamond were Brazil and India, the spherical form was known as the 

 Brazilian type, and the octahedral form as the Indian type. 



Occasionally, only half the faces of the hexakis-octahedron are developed, namely, 

 those occupying alternate octants. The hemihedral form so derived is that of the hexakis- 

 tetrahedron, shown in Fig. 31 h. The faces of this form, which is of rare occurrence, are 

 always curved, smooth, and bright. If the symmetry of the diamond is really tetrahedral- 

 hemihedral, the complete hexakis-octahedron may be regarded as a combination of two 

 hexakis-tetrahedral forms, but the faces in adjacent octants would then have different 

 surface characters, and this has not hitherto been observed. 



