In ancient Chinese painting, such as the world-renowned Dunhuang murals, ancient people often used minerals as pigments, such as hematite (red), cinnabar (red), pyrite (yellow), malachite (green), cuprite (blue), graphite (black), etc., which are commonly used pigments. When a beam of visible light shines on the surface of a mineral, some light is reflected and some is absorbed. If the mineral is transparent and not too thick, some light can penetrate the mineral. When the mineral absorbs all visible light of different wavelengths, it appears black; when it uniformly absorbs visible light of different wavelengths, it appears various shades of gray; and when it hardly absorbs any visible light, it appears colorless or white. If it selectively absorbs visible light of specific wavelengths, it will display the complementary color of the absorbed light, manifesting as coloration.
Why do some minerals selectively absorb specific wavelengths of visible light? There are mainly three mechanisms: First, some minerals contain a class of elements called transition metals, such as iron, manganese, titanium, vanadium, chromium, copper, lead, zinc, cobalt, and nickel. The atoms of these elements can absorb light of different wavelengths, resulting in different colors of the minerals. For example, the bright red of rubies is due to the presence of trivalent chromium ions in the crystal, and the bright green of jadeite is also due to the presence of trivalent chromium ions. Although they are all trivalent chromium ions, they exhibit different colors because the crystal structure also has an impact on the ionic coloration. Another reason is that in the crystal structure of minerals, electrons can transfer between adjacent ions, accompanied by strong absorption of visible light during the electron transfer. For example, in sapphire, electrons transfer between divalent iron ions and titanium ions, selectively absorbing visible light of specific wavelengths. In addition, some minerals have structural defects in their crystal structures, and the absence of ions occupying vacancies selectively absorbs visible light of specific wavelengths. This is why fluorite often appears green or purple.
In addition, according to the reason for the coloration of minerals, if the color is determined by the inherent composition and structure of the mineral itself, it is called autochromy; if it is caused by impurities or gas/liquid inclusions, it is called heterochromy; and if it is caused by physical optical effects, it is called pseudochromy.