Minecraft wiki entry for Mining, associated text: Although the yielded amount of cobblestone, dirt, sand, coal, iron, and to some extent gold, will be enormous, and the possibility of lava is very high, this is not the best method for mining diamond and lapis lazuli, because they're found only at the bottom of the map. (Note: Diamonds and lapis lazuli can be mined using this method, but due to their rarity, the placement of the quarry along with its size will determine the amount.) This type of mining is suggested for getting large amounts of cobblestone, iron and coal. Besides the ability to gain large amounts of resources, this type of mining may expose many caves for further exploration and mining.