DIAMOND: CRYSTALLINE FORM 123 



A regular or twin intergrowth between two hemihedral crystals frequently takes 

 place and results in the production of a holohedral form. The twin intergrowth of two 

 hexakis-tetrahedra gives rise to the twinned crystal, shown in Fig. 31 I, in which for the 

 sake of clearness the edges are represented as straight instead of curved lines. The two 

 crystals interpenetrate at right angles, and the sharp corners, a (Fig. 31 h), of one individual 

 project from the obtuse corners, b, of the other, the faces of the two interpenetrating 

 individuals thus forming re-entrant angles. The sharp projecting corners of such a group 

 are not always present, usually being truncated, as is indicated in the figure ; the truncating 

 faces of each individual belong to the tetrahedron, and are never curved but always per- 

 fectly plane. The truncation, shown in Fig. 31 I, is only slight, while that of Fig. 31 m, 

 is more pronounced. These eight truncating faces together complete the octahedron, the 

 faces of which are plane, as is shown in m and n of Fig. 31, while its edges are replaced by 

 re-entrant grooves, formed, as explained above, by the interpenetration of two hexakis- 

 tetrahedra. On observing the figures it will be seen that these grooves are striated in the 

 direction of their length. The size of the grooves depends on the degree to which the 

 corners of the hexakis-tetrahedra are truncated by the faces of the tetrahedra. When the 

 truncation is a maximum the grooves will be completely absent ; but an octahedron of 

 diamond in which such re-entrant grooves are not to be seen is a rarity. An octahedron of 

 diamond with the sharp edges of the geometrical form must be considered to be the same as 

 shown in Fig. 31, m and n, in which the truncation has quite obliterated the grooves ; in 

 other words, it is the limiting form of such twinned crystals. 



This twin intergrowth is simpler when the two individuals are tetrahedra instead ox 

 hexakis-tetrahedra, as in the case considered above. Such a twin-crystal is shown in Fig. 31 p, 

 where the projecting portions, removed by truncation, of the two interpenetrating 

 tetrahedra ai-e represented by dotted lines. Here the grooves are quite straight, and of the 

 same width throughout, and they do not show the nick in the middle as in the previous 

 twinned form, in which also the grooves widen out away from this nick. 



This simpler twinned form, consisting merely of two interpenetrating tetrahedra, is, 

 however, of very rare occurrence in diamond. On the other hand, the form consisting of 

 two interpenetrating hexakis-tetrahedra, as shown in Fig. 31 m, is very characteristic of 

 diamond, and is of frequent occurrence. This figure has therefore been drawn again in 

 Fig. 31 n, the dotted lines having been omitted and the characteristic markings on the 

 faces inserted. The small faces of the hexakis-tetrahedra, which form the re-enti"ant grooves 

 due to the twinning, are always somewhat curved and exhibit a delicate striation in the 

 direction of their length. A slightly different form of such an interpenetrating twin of 

 octahedral habit is shown in Fig. 31 o ,■ this also is a frequently observed form of diamond 

 crystal. Here the edges of the octahedron have, in place of grooves, two small planes 

 meeting at a very obtuse angle in a short edge at the middle of, and perpendicular to, the 

 octahedral edge ; and away from this short edge formed by their mutual intersection they 

 gradually widen out. These small planes are curved and finely striated, as shown in the 

 figure, the ocfahedral planes being as before perfectly plane. 



The twinned forms just described (Figs. 31 m, n, o,p,) are very characteristic of diamond, 

 and they constitute the octahedral or Indian type. Crystals of this kind, of which one in 

 its matrix is represented in Plate I., Fig. 2, are sometimes known in the trade as " points." 



It has already been pointed out above that while the faces of the rhombic dodecahedron 

 and of the hexakis-octahedron show a convex curvature, those of the octahedron are plane 

 and even. The octahedral faces are, however, characterised by the presence of striations 

 and pits, both of which are repeated on the surface with definite regularity and have a 



