Minecraft wiki entry for Mining, associated text: The results indicate what is expected — that when tunnels are close together they are not efficient because the miner will encounter diamonds which were already removed by the adjacent tunnel(s). A good efficiency is reached at a spacing of around 6 blocks (that is, 6 solid blocks left in-between the tunnels). At this spacing, efficiency is about 0.017, corresponding to 1.7% of blocks removed being a diamond. At this spacing, the tunnels effectively become independent of each other and so, statistically speaking, the chance of encountering an ore are maximized because there is no chance the ore has been removed by an adjacent tunnel.(true independence would need spacings more than a chunk) Above a spacing of 6, efficiency does not increase greatly because ore collection rate is simply a function of the distribution of ores within the level. Note: in the above graph, efficiency appears to drop-off at a spacing of 10. This is simply a limitation of the size of the level used to model the process, resulting in a large error at high spacings. If a larger level were used, the line would smoothly approach a maximum efficiency but never quite reach it.