Steps to reproduce:

1. Turn the UI Profile to 'Classic'.
2. Press 'Trade' to any villager. (angle the camera just enough to see above the villager's head)

You should see an image of an emerald above the villager's head (this does not appear above the other villagers' heads when walking around). This will only appear when you bring up the trade display with the villager you choose to trade with.

I don't know if this is affected with multiplayer turned on (if other players are able to see the emerald when another player is trading).