Prices for rough and polished diamonds are available in Bloomberg. Useful codes are PLPH, RCDI and DIAM1CRT INDEX, for a Rapaport Index.
There are diamond mine reserve volumes available using the Data Tools in Fitch Connect. Select Data Tools in the top navigation menu. Choose the Interactive Charts option. Choose a location using the select entities/geographiesoption. For the data item choose Industries > Mining > Diamond. Next, select a date range and finally a visualization option.