A lyrium vein is an ore vein containing lyrium. There are rich deposits of lyrium veins beneath the Frostback Mountains which are extracted by the Miner caste of the dwarves, who have a natural resistance to their effects. They can also be found in the Fade.
When touched, Lyrium Veins replenish the health and mana of non-dwarf humanoids. This is slightly strange, considering that the codex entry foretells burns, delirium, and death from contact with raw lyrium.