Found 40 diamonds in about an hour and a half, just using single branch mines at level 12. The combined efficiency of mining at level 12 and the large amount of ore in the SouthWest quadrant of the map allows this. Use debug mode to find you're coordiantes. You should to these coordinates for safe measure on any map: 500:12.666666:500.
Your mind=BLOWN
Yes, it is true... there are more diamonds in the south west, BUT, it is south west from the coordinates 0,0... not from where you currently are.