They only serve two purposes: You can craft them into emerald blocks and use them as decoration, or (the much better use for them) you can trade them with NPC villagers for various items. Different villagers will give you different things, from enchanted gear (but you have to also provide them with the gear to enchant) to bookshelves, and even bottles o' enchanting, which can only be gotten through villager trading or through creative mode.
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You have to use a Iron Pickaxe to dig Emeralds ( Found in Extreme hills biome )
dimond
They are only in the 1.3.1 update and you have to look for them like you have to look for diamonds :P
If you are asking if you can just find weapons lying around in Minecraft, that is impossible. The two ways to get weapons in Minecraft are by, 1. Crafting them in a Crafting Bench, and, 2. By trading Emeralds to Villagers that are offering them.
you can either find them in extreme hills biome and extreme hills edge biome or you trade them with villagers. Answer from Andy.