Many people refer to these basalt rocks as crystals. But, basalt is a rock, not a mineral, and therefore does not have a crystal structure of it's own. During the cooling of a thick lava flow, contractional joints form. If a flow cools relatively rapidly, significant contraction forces build up resulting in the formation of columns. The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small columns, while slow cooling is more likely to produce large columns (like these shown).