Why do diamonds twinkle?

Diamonds are cut and polished geometrically in order to reflect and refract light. A diamond's ability to 'twinkle' is one of the four characteristics one examines when buying a diamond: its name is cut. Others are clarity, colour and carat weight.

A tiny change in light angle, or position of the diamond, will alter the light path through the gem (caused by the gem's internal matrix and its external angles). To the observer's eye, this light will fluctuate in brightness (twinkling), as starlight does when refracted by the atmosphere.