Veins are named for their resemblance to blood vessels, which are tubes. But the veins in rocks are actually sheets or tabular shapes, which look like tubes only because outcrops expose them in cross section. A vein is a fracture that is filled with hydrothermal mineral material.
This complex vein is exposed in the ceiling of a former mercury mine in the New Almaden district near San Jose, California. The vein is clearly a fracture because it doesn't fit in with the structure of the rock around it. But the multiple stripes of white and dark minerals suggest that the fracture was active for some time while mineral fluids were rising from below. Each episode of widening and filling the vein may have corresponded to a large earthquake, this locality being near the San Andreas fault.