Wow I found another 2 on layer 10 thanks guys! So far I have a Sword, Pick, and a bunch of other tools in diamonds. I've had the game for 10 days and I've already foun 17 diamond with additional diamond tools.

I haven't found any diamonds and I've had the game for months.

the way i search for diamonds is, i tunnel down periodically checking F3 to keep tabs on my elevation, once i get to layer 10, i start mining in different directions and on other levels (highest level of 16)
diamonds can occur at levels lower than 10, but my experiences tell me about levels10 - 16 are the best for mining through,
gold is about the same only you can go all the way up to layer thirty in mining for gold from layer zero (or ten if you take my advice)

good luck