from minecraft wiki:
It can be found in a vein of 1-8 blocks (with larger concentrations obviously being more rare). Diamond shows up at elevations ranging from 1 to 19 blocks above the bottom of the map (for comparison, sea level is around 64 blocks above the bottom of the map). Below elevation 18, it occurs in roughly 0.08% of rock. This means that you must check 867 blocks below elevation 18 to have a 50% chance of finding at least one diamond ore.