What's the best level to mine Emerald Ore?

Solution 1:

As is written here, you can find Emeralds between 1 and 30. But I don't think it's worth the time and materials you need to mine for Emeralds, since they are 1/40 times as common as diamonds.

Emerald Ore is approximately 1/40 as common as Diamond Ore (174 Emerald Ore in 2382 chunks, compared to 6637 Diamond Ore).

I think a better way to get Emeralds is to sell stuff like, coal (since you can use charcoal for your furnaces, or you can try sell charcoal, but I'm not sure that it will work.), wheat (since it's renewable.), wool (since you can build a sheep farm.) or rotten flesh (if you have a zombie grinder.).