Where can I find diamonds?

In my experience diamond is often found very deep underground. Usually I find it in large caves near lava or water. You definitely have to be really deep to find it though, I'm pretty certain of that.

Try looking in the ground...

Agree with Cyrus and Mewtie. From my best encounters, find a HUGE natural cave and explore around. I've hit 5 deposits (totaling to 15 diamonds) in a cave. Still only about half explored too

Ok here is your best bet for finding just about anything in the game. What you do s dig a safe tunnel down to debrock which is industructable then you go up 5 levels above the highest bedrock you find and that will be your floor, from there make a small base room and take whatever kind of pick you feel like using and dig a tunnel just big enough for your charector to fit through as far in as you want and then go all the way back toy uor base room move 5 blocks over and try again i have found like 128 iron, a whole ton of coal, around 64 gold, and at least 50 dimond this way. and yes it will take a little while but its worht and another advantage to this is that the height you are at is where lava will not spawn above or next toy ou only below you so you are safe form lava.