keep in mind that there are rampant cheaters in this game using an exploit so be prepared to play against 'perfect opponents'. although as far as i can see from my friends list, those people get banned at some point

your best bet is to try doing this with a friend

diamonds are for the new holi-duels, in order to play a match you have to spend a diamond. If you win, you get a diamond. You get a free match a day. it's like a head to head tournament. there are different things you get like an avatar border, for example winning 3 in a row at the emerald level gets you an avatar i believe. winning the whole tournament will get you one too. they are like the custom dice.