I've found a way to replace a certain percentage of a particular block (ex. stone or gold ore) with emerald using worldedit. //replace 14 75%14,25%129 or //replace stone 99%stone,01%129 for example However, Worldedit cannot handle selecting an entire world. This is a request for an external program or plugin that will scan my world/chunk files themselves and edit them from outside them game. Or a plugin that will check chunks when they are loaded to replace a certain amount of X ore with emerald ore and then remember which chunks it has modified so it doesn't look at them again. This would allow for existing 1.2.5 worlds to essentially seed emerald ores throughout established areas.