We examine possible mechanisms for graphitization at dislocation cores in d
iamond, both as an explanation for brown coloration in natural brown diamon
d and as a model for annealing in amorphous carbon materials. We find a met
astable structure for the 90 degrees partial dislocation core where bonds a
re broken in both single- and double-period structures leading to a row of
paired sp(2) c atoms. We examine the effect of hydrogen and other impuritie
s on this core structure both through direct interaction and potential char
ging effects. We also investigate a mechanism for progressive aromatization
through the migration and aggregation of 30 degrees partial dislocations.