Mineral vein
Mineral veins are veins of precious stones or ores that can be found on walls of some mines. They are just like regular Mining rocks, but they are found in cave walls rather than in outcrops on the floor. There are several different locations that veins can be found in, most notably in the Keldagrim mines, the Arzinian Mine and the Abandoned Mine in Morytania. They can each contain different types of Ore, except in the Arzinian Mine which only Gold ore can be mined out of the veins. To get to the veins at Keldagrim you have to take a ship from the Dwarven Ferryman for free and walk west from the ship.
Another example of mineral veins would be the Daeyalt mine deep underneath Meiyerditch, the slum-capital of Morytania, though Daeyalt is found as nuggets embedded in the walls, not in veins.