It seems that the emeralds used for trading won't be found in ores.
Quoting Jens from reddit (jeb_ comments on Jeb removing emerald ore again
The reason I added emeralds in the first place was because I wanted to add a currency that I can "control." In other words, something that's exclusive to trading.
Then I decided that the emeralds could be found in really small amounts in the "extreme hills" biome, just as a nice surprise for people who mined there. The amount was really small, and it was really hard to find since it's were spread over a larger y-span. In other words, not worth the effort to search for, but a nice surpise if you happend to find it.
This lead to the situation where I received a lot of complaints that it took too much effort to find emeralds. I could of course ignore those complaints, but it's very annoying to see it popping up everywhere. I could have "fixed" that by making it easier to find, but then I would've ruined the basic idea to keep emeralds linked with trading.
So, now I decided to remove the ore from the world generator. This will hopefully lead to less confusion, and fewer complaints in the long run.