Minecraft wiki entry for Old_Customized, associated text: Each of the eleven sections of this page modifies the generation settings for each type of ore. These settings can be used to change the distribution of almost all any ore and mineral type. The only exception is emerald ore, as it generates only in extreme hills biomes, and the blob size is always one block; as its distribution is hard coded, it cannot be changed. The distribution of natural resources in the Nether (Nether quartz ore and magma block) also cannot be changed.