The cause is unknown; however it is thought to be associated with sun exposure, leading to a dilated blood-filled vascular channel "...lined with a singled layer of flattened endothelial cells and a thin wall of fibrous tissue filled with red blood cells."
Venous lake
Venous hum
Necklace of Venus
Venom
Phrygian cap 
Bleb 
Arterial tree
Aura