What does 'play-of-color' specifically describe?

The term 'play-of-color' specifically refers to the phenomenon observed in opals, where the stone exhibits a unique and captivating display of shifting rainbow colors as it is moved under light. This optical characteristic occurs because of the internal structure of opal, made up of microscopic spheres of silica that diffract light, creating a spectrum of colors that vary with the angle of observation. This dynamic interaction of light and the stone's internal composition is what distinguishes opals and contributes to their value and beauty in the world of colored gemstones.

While the other options discuss aspects related to gemstones, they do not pertain specifically to 'play-of-color.' For instance, the sparkling effect in diamonds, the color change under different lighting, and the brightness of polished surfaces describe different visual effects or properties that do not match the distinct, colorful play seen in opals.