The Colors in Cabochons
Mineral colors are primarily provided by transition materials (metals) acting as impurities (allochromatic) or as essential components (idiochromatic) within the crystal lattice. Key coloring elements include iron (red/yellow), aluminum/titanium (blue), copper (green/blue), chromium (green/red), manganese (pink), cobalt, and nickel, which absorb specific wavelengths of light.
Minerals have been the source for color pigments in paleo-lithic cave paintings since early humans roamed the earth. What gives minerals their fantastical colors? It all boils down to four reasons:
Chemical composition
Chemical impurities
Inclusions of other minerals or elements
Environmental impact
Chemical composition: Certain elements (particularly heavy metals) strongly absorb portions of the light spectrum and therefore appear to have certain colors. Minerals that are based on these elements will be consistently “self-colored” or idiochromatic. Although still subject to the effects of trace elements, the mineral’s color is constant and predictable.
Copper in different oxides and in different crystal forms produces the teal blue in turquoise, the azure-blue color of azurite, and the vivid green in malachite.
Manganese produces the pink color of rhodochrosite and mercury makes cinnabar which can make it toxic if the level of the mercury is too high. The violet red of erythrite is from cobalt.
Chromium produces the orange-red color of crocoite, but the red-orange color of vanadinite is caused by its Vanadium.
Chemical Impurities
All natural minerals contain minute impurities. This means that in addition to their chemical composition, they have trace quantities of other elements (often less than 1% of their mass) that have been incorporated into the mineral as it formed.
Many minerals get their color from impurities and are called allochromatic. Common coloring elements include titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper.
As the amount and type of impurities varies, so do the colors within a mineral. For example: pure quartz is colorless, but add some iron impurities and it turns into purple amethyst. Add slightly more iron and you get yellow citrine. Mix some trace amounts of titanium or manganese to pure quartz and you have pink rose quartz.
Mineral Inclusions
Inclusions of one mineral within a host mineral can also cause a color difference. Quartz has many examples of color changes due to inclusions. Chlorite inclusions in quartz cause it to turn green. Rutile inclusions give it a golden hue. Milky quartz is the most common form of quartz and gets its color because of minute fluid inclusions of gas and/or liquid trapped during the crystal formation.
Environmental Impact
Certain minerals change color when exposed to sunlight, heat, or radiation. Red realgar transforms into yellow pararealgar upon repeated exposure to sunlight. Some minerals, such as proustite and vivianite, darken upon prolonged exposure to light. whereas other minerals, such as Kunzite will fade when exposed to too much sunlight.
Some minerals change color when heated (a common technique to artificially enhance the color of many gemstones.) Naturally occurring or artificially applied radiation can also have an effect – smoky quartz being the most common example.