What phenomenon occurs when a gem's crystal structure splits light into two rays?

The phenomenon that occurs when a gem's crystal structure splits light into two rays is known as double refraction. This occurs in materials that possess an anisotropic crystal structure, where light travels at different speeds depending on the direction in which it moves through the material. When light enters such a gemstone, it is refracted, or bent, in two distinct ways, resulting in two separate rays of light.

Common examples of gemstones that exhibit double refraction include calcite and some varieties of quartz, where you can noticeably see a double image when looking through the stone. This characteristic is vital in gem identification and adds to the visual complexity seen in certain precious stones.

The other options describe different optical phenomena but do not specifically refer to the splitting of light into two rays as observed in double refraction.