The crystallography of diamond presents problems of peculiar interest and difficulty. The material as found is usually in the form of complete crystals bounded on all sides by their natural faces, but strangely enough, these faces generally exhibit a marked curvature. The diamonds found in the State of Panna in Central India, for example, are invariably of this kind. Other diamonds as for example a group of specimens recently acquired for our studies from Hyderabad (Deccan)show both plane and curved faces in combination. Even those diamonds which at first sight seem to resemble the standard forms of geometric crystallography, such as the rhombic dodecahedron or the octahedron, are found on scrutiny to exhibit features which preclude such an identification.

This is the case, for example, with the South African diamonds presented to us for the purpose of these studies by the De Beers Mining Corporation of Kimberley. From these facts it is evident that the crystallography of diamond stands in a class by itself apart from that of other substances and'needs to be approached from a distinctive standpoint. It is essential, at the very outset, to emphasise the point seemingly obvious but often overlooked-that a crystal which exhibits curved faces cannot properly be described in the usual terminology which is based on the existence of plane faces obeying the crystallographic law of rational indices. One of the most firmly established results of physics is the dependence of the physical properties of a crystalline solid on the symmetry of its structure of which the external form is an indication. There can be little doubt, therefore, that a study of the crystal forms of diamond, pursued from an appropriate standpoint, would prove most. helpful in understanding and interpreting the many remarkable properties of this substance. These considerations and'the availability of the material referred to above some 72 specimens in all encouraged us to undertake a critical examination of the subject.

The investigation had for its object the discovery of the factors determining the general shape and other distinctive features of the crystal forms, and of the connection between them and the internal architecture of the crystal. The studies have enabled us to establish some definite propositions concerning these matters which are stated below. The evidence on which our conclusions are based will be set out fully in the course of the paper.

Contents
1. Introductory statement
2. General descriptive characters
3. Some theoretical considerations
4. Geometric preliminaries
5. The configuration of the edges
6. The crystal symmetry of diamond
7. Classification of the crystal forms
8. The Haidinger diamond
9. The triangular twins
10. Some descriptive notes
11. The allotropic modifications of diamond
12. Summary. References. Plate

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