124 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



definite orientation. The striations are parallel to the symmetrical six-sided outline of the 

 octahedral faces, as shown in Fig. 31 o, for the whole crystal, and in Fig. 31 s, for a single 

 face. They may be either coarse or fine in character, and may be present in small or large 

 numbers. The portions of the face of the octahedron between the striations are often very 

 smooth and bright. These striations are due to the fact that the octahedral face is raised 

 by very low steps towards its centre, each step having the same sharp outline as the margin 

 of the octahedral face itself. It is as if numerous very thin plates, all of the same shape 

 but gradually diminishing in size, had been piled, with their centres exactly superposed, on 

 the octahedral face, so that each layer forms a step, and so a line of the striation. 



The triangular pits are regular pyramidal depressions, of which the bases are equilateral 

 triangles. They are usually small and often only to be seen distinctly under the microscope. 

 The pyramidal faces inside the pits are finely striated and may terminate in the apex of the 

 pyramid, as shown at a in Fig. 31 q ; or they may not extend to such a depth into the 

 interior of the crystal, the apex of the pyramid being then truncated by a triangular face 

 parallel to the face of the octahedron, as at b. Fig. 31 q. Sometimes on this inner face 

 there is a smaller pyramidal depression as at c, Fig. 31 q. These depressions are of the 

 same general character as those produced on the octahedral faces of a diamond during its 

 combustion ; but while the corners of the pits of natural origin are adjacent to the octahedral 

 edges (Fig. 31 n, o, q), this position is occupied by the sides of the pits produced by etching 

 (Fig. 31 r) ; thus the two positions are the reverse of each other. The pits occur singly or 

 in large numbers, and the striations may or may not be also present on the same face 

 (Fig. 31 o, n). 



Beside, the twin-crystals formed by the interpenetration of two hemihedral crystals, 

 illustrated in Fig. 31, Z to p, diamond presents stiU another type of twin-crystal, which is 

 illustrated in Fig. 31, g to i. Here two octahedral or rhombic dodecahedral crystals are 

 united together along a face of the octahedron. Fig. 31 g, shows two octahedra symmetri- 

 cally united in this manner, the two individuals having one octahedral face in common. 

 This kind of twin-growth is frequent in diamond but still more so in the mineral spinel, so 

 that the law which governs this kind of twinning is referred to as the spinel twin-law. At 

 the line of junction of the two individuals, three re-entrant angles alternate with the same 

 number of sahent angles. These spinel twins of diamond, which are known to the diamond- 

 cutters of Amsterdam as " naadsteenen " (suture-stones), are very frequently flattened in a 

 direction perpendicular to the common octahedral plane ; they are, indeed, sometimes 

 reduced to mere thin plates, but the faces and edges always have the surface characters 

 described above. 



Very frequently two rhombic dodecahedra or two hexakis-octahedra are twinned 

 according to the same law on a face of the octahedron ; that is, the two individuals have a 

 face in common which occupies the position of an octahedral face, and about which they are 

 symmetrical. These twin-growths also are much compressed in a direction perpendicular to 

 the twin- plane; this is illustrated, in Fig. 31 h, which represents a lenticular or heart- 

 shaped crystal with curved faces. In this crystal only six faces of each of the hexakis- 

 octahedra are developed, and these form low six-sided pyramids with a common base parallel 

 to the six-sided octahedral face shown in the figure. 



Fig. 31 i represents another kind of twin growth of rarer occurrence, in which the 

 crystal has the form of a rhombic dodecahedron (Fig. 31 c). Parallel to one or more of the 

 possible faces of the octahedron, which if present would truncate the corners in which three 

 edges meet, are very thin lamella; in twin positions to the main crystal. Large numbers 

 of these twin-lamellas may be present, and give rise to striations on the faces of the crystal. 



