You guys using emeralds as a new feature or emeralds as the old name for diamonds? It's kind of loving confusing you guys started using emeralds.
New feature. They're used as currency in villages.
Adventure game mode doesn't let you break blocks.
what
That's the point.
Survival = Build a shelter, mine, etc.
Adventure = Go on adventure. Raid towns, open chests, solves puzzles that use levers, etc. It's like in most adventure maps when you're not supposed to break any blocks.