CS Book 1_AS- 4 Gem Light.pdf

Gems and Light KEY CONCEPTS Color is the most important contributing factor to a colored Colors produced by charge transfer are much stronger than stone’s desirability, mark...

Gems and Light KEY CONCEPTS Color is the most important contributing factor to a colored Colors produced by charge transfer are much stronger than stone’s desirability, marketability, and value. colors caused by dispersed ions. Light is a form of radiant energy that travels in waves. Treatment can alter or initiate some color-causing charge transfer reactions. Visible light is a small portion of the electromagnetic spectrum. The presence of transition elements adds new energy levels within a gem’s band gap, helping electrons absorb some White light is a combination of the spectral colors red, orange, visible light wavelengths. yellow, green, blue, and violet. Most gem color is caused by selective absorption, which is Irradiation produces color in gems by creating color centers. influenced by chemical composition and crystal structure. Color centers allow selective absorption to take place by Interaction of electrons with visible light causes most selective lowering the energy required for electrons to make transitions absorption related gem colors. in the band gap. Chromium causes the most desirable red and green gem colors. Heat treatment removes unwanted colors by destroying the color centers that cause them. Iron causes a greater variety of gem colors than any other transition element. Only doubly refractive gems can be pleochroic. Valence differences can cause variations in color intensity in the same gem material. Dispersion is greater in hi-RI gems than in low-RI gems. Crystal structure variations result in the same transition Interference produces either darkness or the brightening of element causing different colors in different gems. one or more colors. 189