COAL

Coal is a raw-material trade: villagers pay emeralds for it, which makes it a candidate for an automated emerald farm rather than something you purchase back.

Coal appears in the villager trade economy as a tradeable good.

It passes through 4 different professions — Armorer, Butcher, Toolsmith, and Weaponsmith — so several trading-hall layouts can supply it. Coal can also be obtained outside of trading, but buying it from a villager is often faster than gathering or smelting the ingredients yourself.

To sell Coal for emeralds, bring it to an Armorer at the Novice tier (15 units per 1 emerald), a Butcher at the Apprentice tier (15 units per 1 emerald), a Toolsmith at the Novice tier (15 units per 1 emerald), and a Weaponsmith at the Novice tier (15 units per 1 emerald).

On the supply side, Coal is worth one emerald per 15 units at its best rate (the Armorer). That ratio is what decides whether farming coal for currency is efficient: the fewer units a villager demands per emerald, the more valuable each one is to produce. A buy slot like this can be used 16 times before it locks and the villager has to restock, capping how many emeralds one villager returns per cycle before it must work at its job site again.

Since no villager sells coal back, this is a one-way emerald faucet: you supply the coal, the villager supplies the emeralds. That makes it a prime candidate for an automated trading hall, where a renewable source of coal feeds a row of villagers and steadily converts your surplus into currency you can spend on the gear and rare items other professions sell.

Every Coal trade follows Minecraft's standard supply-and-demand rules. A trade can be used a limited number of times — 16 for the slot in question — before it greys out and locks until the villager restocks. A villager restocks up to twice per in-game day, but only if it can path to its job-site block and "work" there, which is why a trading hall must leave the workstation reachable.

When you sell coal to a villager, completing that trade is what grants the villager career experience and pushes it toward its next tier. Spreading sales across several villagers of the same profession multiplies both your emerald throughput and how quickly the hall levels up.

One caution: the first trade you complete with a fresh villager permanently locks its current offers, including the coal slot at whatever price it rolled. If you are chasing a specific price, decide before you trade, because there is no re-rolling a villager once it has done business with you.