Colors of Coral

Viewed in polarized light, this thin section of the skeleton of a Pacific reef-building coral, Acropora gemmifera, looks more like abstract art. The skeleton is made up of millions of tiny crystals of calcium carbonate that behave like prisms and refract the light from the microscope. For scale, the width of the black holes on the skeleton are roughly 100 microns. (Photo by Anne Cohen, Woods Hole Oceanographic Institution)