The origin of color in minerals in general and gem minerals in particular has been investigated with increasing sophistication in recent years. Moreover, recent concerns about gemstone enhancement, which usually implies color enhancement, make it necessary to understand what happens during treatment and what changes in appearance can and cannot be achieved.
A strong interest in minerals arose early in history, because they are among the few natural materials that can permanently retain color, unlike flowers and plants, which fade or change with time. Colorful minerals such as iron oxides became the basic pigments of early paintings. Eventually, colored stones were cut and polished so they could be worn for adornment. From antiquity into the 18th century, color was also at the center of numerous superstitions, legends, and even "medical" treatments involving gems and minerals. Yellow stones, for example, were supposed to cure jaundice, and green stones were believed to soothe the eyes (Kunz, 1913).
As mineralogy became a science at the turn of the 19th century, color was used as a common indication in the identification of minerals. Soon, though, people discovered that crystals of the same mineral species could vary in color, and they began to surmise that some hues were related not to the mineral but to specific impurities (figure 1).Goethe, for example, was one of the first to relate the amethyst coloration in quartz to its iron content. The simple correlation between a certain color (e.g., "emerald green") and a given element (chromium)works to some extent, but modern research has shown that a number of very different processes can result in a similar color .
Five mechanism for the origin of color have been proposed by researchers as follows:
(1) Color caused by dispersed metal ions;
(2) Color involving multiple atoms;
(3) Color involving color center;
(4) Color originated from band gaps;
(5) Color caused by physical phenomena;
Color Origin of emerald
Color Origin of Ruby and Sapphire