Which phenomenon is described as diffraction?

Diffraction is the phenomenon that occurs when light encounters an obstacle or a slit that is comparable in size to its wavelength. This interaction causes the light waves to bend and spread out, resulting in the separation of white light into its individual spectral hues. This process is distinct from other optical effects, as it emphasizes how light can interact with materials at a microscopic level, leading to the colorful patterns often seen in diffractive materials such as opal or certain types of coatings on other gemstones.

While the other choices involve relevant concepts in the study of light and gemstones, they do not accurately define diffraction. The splitting of light into two rays pertains more to the phenomenon of birefringence, a property of certain crystals rather than diffraction. Treatments to enhance color generally refer to processes applied to gemstones to improve their visual appeal, which involves external methods rather than a physical property of light. A type of gemstone cut relates to how a stone is shaped and faceted, which is a matter of design rather than an optical phenomenon. Thus, the correct answer captures the essence of how diffraction specifically deals with the separation of light into its spectral components.