Because of its highly uniform and tightly latticed carbon structure, diamonds dramatically slow down light passing through it. Light travels in a vacuum at about 186,000 miles per second, while diamond slows that down to about 80,000! By comparison, glass drops light speed down to about 120,000 mps. Light passing through a diamond gets severely bent upon entry, allowing it to bounce around inside and disperse. This is what causes that diamond sparkle we’re all so familiar with!