You may as well go to a medical forum and ask how to preform open heart surgery.
"How hard can it be." Right?
The real diamond industry and the goods are all locked up behind networks of connections and longtime relationships.
If you have to ask here where to sell rough diamonds you're wasting your time and probably grossly overpaid for what are likely misrepresented stones ... that's IF they are even natural untreated diamonds.
As Kenny said, the diamond supply chain and market are not set up in a way that allows small investors to participate on equal footing with the big guys. Anyone who tells you otherwise is probably getting ready to scam you.