It depends on what time you play. I have my times for when I buy stuff and sell stuff. Sometimes I see diamonds go for about 40-50 grass (only a couple of times) and sometimes I see diamonds being sold for roughly 10-15 grass. Yes these are the extreme prices that I have seen before but it all depends on what time you play. I'm guessing it's because of supply and demand but I'm not sure why it changes so much.