Detailed, quantitative electron backscatter diffraction (EBSD) analysis of polycrystalline diamond rocks (diamondites) provides insights into their formation and significance. The fine grain size of diamondites is usually attributed to rapid crystallization. However, EBSD reveals significant intragrain bending, distinct low-angle boundaries and straight to highly irregular high-angle grain boundaries. Highly deformed grains may be in contact with others showing little or no deformation. These features are typical for crystal plastic deformation, in which differential stress generates dislocations according to the dominant slip systems in diamond. Dislocations accumulate to form subgrain boundaries, with grain-size reduction by rotation of subgrains, nucleation and growth of new grains, and migration of grain boundaries. Such features are seen in other polygranular materials such as deformed metals, and quartzite deformed at high temperatures. During this process, interaction with fluids produced interstitial garnets with compositions different from those of the primary inclusions. Oscillatory CL zoning in diamonds developed through diffusion along subgrain boundaries, rather than being a primary growth feature. Diamondites are thus not simply products of primary crystallization, but may be strongly deformed, recrystallized and modified. Integrated EBSD, CL and in situ chemical and isotopic analysis provide the spatial control and new insights into mantle processes.