If you rename emeralds to something, then try to click on the trade (on the sidebar of the villager UI) of a villager, it won't work unless you put the emerald(s) in the trading slot (the trading slot is where you put the emeralds for a trade).

To reproduce

1. Rename a stack of emeralds (and have no other emeralds in your inventory
2. Click on a villager's trade (where the villager is offering some item for emeralds)
   → Nothing will happen
3. Drag the emeralds into the trading slot
   → The trade will succeed

Video

Minecraft 20w51a - Singleplayer 2020-12-31 16-08-04.mp4