Here is a tutorial on how to mine for diamonds. It is the most efficient way of doing it saving time destroying blocks you don't need to. If you mine the right spot you will find a good amount of diamonds also use a enchanted pick with fortune III and you will get at at most 3 diamonds per block. I also forgot to mention in the video that after you are feed up of mining on that level move up a level and start the same process again until you reach level 16 that is when you stop finding diamonds they only appear from level 16 and under.
Hope the video is useful for you in finding diamonds, Thanks for watching