If you have some time, you can go to Brazil (eg state of Bahia, near Chapada Diamantina where many diamonds are found), talk to people in the villages (Spanish works, Portuguese is better) and buy a raw diamond they found.

Polishing will make it smaller by about half, and at least I cannot judge the purity and value of a raw diamond just like that, but the story and experience of getting it and the knowledge of its ethical origin are enough of offset that in my opinion.

edit: If anyone needs directions, just ask.