Coal is a seasonal item introduced into RUST and is available to players during the Season’s Beatings! event every December. It is spawnable via the F1 console any time of year.
Coal is one of those seasonal items that serves no real purpose other than to fill a void that players would otherwise notice. There’s just something familiar about getting a lump of coal in your Small Stockings and SUPER Stockings. It can be recycled for a minimal amount of resources but otherwise exists because Facepunch enjoys trolling its player base.