Hailstones are not perfectly round.
These are not like snowflakes. These ice crystals melt and freeze during their fall and grow large in the process. Smaller hailstones, though, are usually roundish and without the ugly spikes you see in this image. Smaller hailstones have been cut in half to show that there are different layers and densities inside. This is due to the repeated freezing and melting process. The same process made smaller hailstones bond together.
