Physics of Crystal Colors

Like people, perfect crystals do not exist, and if they did, they would be useless and uninteresting. Color centers such as anion vacancies can be created by energetic particles, a useful effect for radiation dosimetry. Ruby and emerald get their red and green color from chromium impurities. The famous Hope diamond contains boron acceptors that absorb into the red and green region of the spectrum, giving it a striking blue color. Pleochromism is a phenomenon in which a crystal's color depends on which direction you look through it. Most recently observed in gallium oxide, pleochromism is a direct result of the anisotropy of the point-defect environment. Photochromism occurs when a crystal's color changes after exposure to light. Photochromism (or photodarkening) is closely related to persistent photoconductivity and may originate from the same defect physics.