Without the specialized light, the particles pretty much vibrated in place like so many tiny idling engines. When the scientists turned on the light, the hydrogen peroxide and hematite began a chemical reaction that propelled the particles forward.

The scientists watched under a microscope as, at first, the particles moved about at random. Then, about 25 seconds into the chaos, the limited space and directionless driving produced a traffic jam of particles, said study leader Jeremie Palacci, a postdoctoral fellow at NYU.

The jammed particles forced themselves against each other in the pattern of a crystal, each dot surrounded by six others in a hexagonal shape. When they reached a certain size, some of the particles on the edge broke off and grew into other crystals, which slowly moved about.