When the emerald update came out, villagers wanted power, so they thought emeralds would make them powerful. They scowered around, trying to collect all the emeralds they could by trading. After a long amount of time, they collecte most of the emeralds in their world. When they asked Notch what they do, he said "nothing," so they stopped trading for emeralds, but some villagers still don't know this, so they still trade for emeralds. That's why some villagers trade for emeralds and others try to give you their emeralds.
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