What phenomenon describes play-of-color seen only when light travels through an opal to the eye?

The phenomenon that accurately describes the play-of-color seen in opals when light travels through the stone to the observer's eye is known as "contra luz." This term directly translates to "against the light" in Spanish, emphasizing the specific way light interacts with the internal structure of the opal.

In opals, the unique internal arrangement of silica spheres creates a diffraction effect that splits white light into its spectral colors. When viewed under specific lighting conditions, particularly when the light source is behind the opal, the play-of-color becomes visible, showcasing the vibrant hues that make this gemstone so coveted.

While terms like "play-of-light," "backlight," or "direct light" may refer to various light interactions in gemstones, they do not specifically capture the essence of how opals exhibit their captivating color display in relation to light sources. Recognizing the importance of viewing conditions is key for understanding how various gemstones, especially opals, exhibit their unique optical properties.