Iron, for example, colors glass yellow.

Adding a purplish manganese dioxide ore called pyrolusite neutralizes the yellow color in the glass, making the glass clear and color-free.

“As real as color seems to us, it is, in fact, a purely psychological phenomenon. Light rays are not colored; they are radiations of electromagnetic energy of differing wavelengths. The attribute of color is entirely a fabrication of the visual system.”

“Shakespeare found in the opal a symbol of shifting inconstancy, likening play of color to play of mind in one of the most apt uses of gemstone symbolism in literature. In Twelfth Night, he writes, ‘Now the melancholy God protect thee, and the tailor make the garments of changeable taffeta, for the mind is opal.’”