sentences:  A "reef" is another term for a vein of some mineral or ore, and has nothing to do with biological reefs (bioherms), such as coral reefs.Quartz is one of the most common minerals in the earth's crust, and most quartz veins do not carry gold, but those that have gold are avidly hunted by prospectors.Mining the ore usually required mine shafts sunk to mine quartz from the reefs, sometimes deep underground.