RuneScape Classic Wiki
The main RuneScape Wiki has an article on: A lump of Charcoal
The Old School RuneScape Wiki also has an article on A lump of Charcoal.

A lump of Charcoal can be bought from General Stores in Shilo Village or Tai Bwo Wannai, or dug from training sites and specimen trays at the Digsite.

It is used for drawing a map in the Digsite and Legend's Quests, and it is ground up with a pestle and mortar to get ground charcoal for explosive compound.