Personally I think emeralds should be in the market, but with prices like these:
To buy ONE emerald: $3000
To sell ONE emerald: $100
The above prices would hopefully discourage selling of emeralds to the market (thus removing them from the world altogether), and will discourage buying of them, or make it so many less emeralds are purchased thus keeping them a rare commodity. Plus with villager trading the sale price shouldn't be higher than 100 per emerald because people will find ways to trade with their villagers for lots of emeralds.