In Treasure cave, what determines the amount of diamonds?

Your teams rating. The higher the rating, the higher the diamonds

They always change for me, either 2, 4 or 6.

ok if you meant how do u get diamonds after plunder its pretty much random

Oh right. If it's mining then I find the longer you mine, the more you receive, however don't bother paying 50 diamonds to mine a 24 hour mine. I did it once, and I got 55 diamonds, meaning in 24 hours, I only made a profit of 5. Biggest rort ever. Go for the 10 hour mines twice daily, and possibly squeeze in a 3 hour mine between them if you can.

Basically...

- 1 10 hour mine while you're at work - do this when you wake up
- 1 3 hour mine when you finish work and are going home, and are eating dinner/showering etc...
- 1 10 hour mine while just before you go to sleep