Curvature of Crystal Planes (Diamond)
Curvature of crystal planes is quite a constant property of some substances. It may be due to a very fine oscillatory combinations or to an irregularity of growth and a want of perfect parallelism between sub-individuals. A common instance is the Diamond, whose faces, except those of the octahedron, are almost always curved.
George Huntington Williams, Ph.D. Elements of Crystallography for Students of Chemistry, Physics, and Mineralogy (New York, NY: Henry Holt and Company, 1892)