a color center
Why is a flawless, colourless crystal of table salt or sapphire sometimes deeply tinted — smoky, purple, blue — with no coloured impurity added at all? Often the answer is a color center: an electronic defect trapped at a point defect that absorbs a particular wavelength of visible light, so the crystal transmits the leftover colours. The classic one is the F-center (from German Farbe, colour): a single electron caught in the trap of an ANION VACANCY.
The mechanism is neat. An anion vacancy in an ionic crystal carries a positive effective charge (V_O•• or, in a halide, V_Cl•), so it acts like a tiny positive well that can capture a wandering electron. The trapped electron sits in quantized energy levels, much like a particle in a box, and the spacing of those levels happens to fall in the visible range — so the electron absorbs a specific visible photon to jump between levels, subtracting that colour from transmitted light. You can create such vacancies by heating the crystal in the metal vapour (adding excess metal), or by irradiation with x-rays or a reactor, which knocks ions aside and leaves vacancy traps.
Color centers explain a gallery of real phenomena: additively-coloured NaCl turns yellow-brown, natural and irradiated fluorite goes purple, and smoky quartz, irradiated blue topaz, and some yellow sapphire owe their tints to trapped-charge centers, while radiation-darkened optical windows and dosimeters work the same way. In ceramics they are a double-edged sword — a nuisance that fogs a transparent laser or window ceramic and a radiation-damage warning sign, but also the working principle of radiation dosimeters and some tunable-laser and data-storage media. Many color centers are bleachable: warm the crystal or shine light on it and the trapped electrons escape, and the colour fades.
Heat a colourless NaCl crystal in sodium vapour: the excess sodium forces chlorine vacancies, each of which traps an electron to form an F-center. The crystal turns a deep yellow-brown even though it still contains only sodium and chlorine — the colour is pure defect, not impurity.
An F-center is an electron in an anion-vacancy trap; its quantized levels absorb visible light and paint the crystal.
A color center is not the same as impurity colour. A ruby is red from chromium impurity (an added ion), but an F-center colours a crystal with no foreign atom at all — the pigment is a trapped electron sitting in a vacancy.