Coal

Coal is a free-to-play item on the OSRS Grand Exchange in the ores category.

In-game examine text: "Hmm a non-renewable energy source!"

It has a Grand Exchange buy limit of 13,000 every four hours, a high alchemy value of 27 gp, and item ID 453.

View on OSRS Wiki

Coal shows an active flip setup with a margin of +5 gp and 23,644,804 trades over the last 24 hours.

At the buy limit of 13,000 every four hours, max profit per cycle is about 65.0k gp — high margin with healthy liquidity.