Crude oil comes in many forms. Usually it is black, but green, red or brown oils are not uncommon. Thin and volatile oils are called "light", whereas thick and viscous ones are "heavy". Light oils have an API gravity of 30 to 40 degrees, which means that the density is much less than 1.0 g/cc. These oils float easily on water. By contrast, some heavy oils have an API gravity of less than 12 degrees and are so dense that they sink, rather than float, in water.