It has long been known that hardness is a vector property. For many substances hardness curves have been determined showing that this property varies with the different crystal faces of the substance and also with the direction on the faces. That the hardness of the diamond varies materially in this manner has been common knowledge to diamond cutters. Unfortunately erroneous statements concerning the variation of the hardness of the diamond have found their way into mineralogical literature, and only in recent years have efforts been made tocorrect these statements. Significant contributions to our knowledge of the hardness of the diamond and its relation to crystal structure and to cutting for gem and industrial purposes have been made by W. Fr. Eppler, H. Rose, K. Schlossmacher, and more recently by H. Bergheimer. It is the purpose of this paper to discuss the variation of the hardness in the diamond, as established by the experience of diamond cutters, in terms of crystallographic theory, crystal structure, valence bonds, cleavage, and growth and solution phenomena.