YD there is no realistic answer to your question, diamonds are a...

YD there is no realistic answer to your question, diamonds are a difficult commodity to deal with and understand.
Exploration is fraught with difficulties.
Unlike gold you can't just go on volume grade as diamond valuation within pipes plays a huge roll, Argyle was successful at $25/ct as it was a large pipe with high grade, most pipes are uneconomic below $50/ct. The avg price is around $120/ct some go as high as $1500/ct.

The Ellendale alluvials may prove uneconomic, though I would suggest this is unlikely. The diamond pipes may not get back to production?
Canada may be a bust economically and they may find nothing in Botswana. Downstream marketing may not work.

Back a company that has diamond related skill-set particularly mining/exploration.
Look at the location of exploration, some areas are more conducive.
Usually you want more than one exploration option as you will get more misses than hits.
If you get a hit it can be highly profitable.
Bit of luck helps!