Basalt is a gray to black igneous rock that is mostly commonly formed by the cooling of lava. Its shape or rock formation is determined by the how and where of a volcanic eruption. Lava cooled underwater forms pillow-shaped basalt rocks. Fast flowing lava creates jagged rocks known as Aa basalt. Glassy rope-like sculptures, called Pahoehoe basalt, are formed when lava flows beneath a rock surface. Thick lava flows cooled quickly by air shrink, crack and form columns. Basalt columns, also known as columnar basalt, are often said to be hexagons yet these polygons can have any number of sides.