First it helps to understand something about the physics of color. Basically, visible light is a form of electromagnetic radiation, just like radio waves, microwaves, X-rays, and gamma rays. If we picture these as waves, the wavelength is the distance from the peak of one wave to the next. Gamma rays and X-rays have very short wavelengths - far too short for us to see. Slightly longer waves give us ultraviolet (or UV) light, and past UV light we come into the realm of visible light, starting with violet, which has the shortest wavelength we can see. As the wavelengths get longer, the color shifts from violet through indigo to blue, into green, etc. Red has the longest wavelength we can see, above which comes infrared, then microwaves and radio waves.
White light, such as sunlight, contains all the colors of the visible spectrum (and in the case of sunlight, also some infrared and UV, which is why we wear sunblock). The accepted theory of color is that some of the wavelengths of incoming light are absorbed by the colored object, and that the remaining wavelengths determine its final color. We'll take a closer look at the mechanism for this in just a minute.
Minerals are divided into two classes, based on what causes their color:
- Idiochromatic - minerals whose color is determined by a coloring agent that is a regular part of the ideal chemical formula.
- Allochromatic - minerals whose coloring agents are not part of the ideal chemical composition.
Some minerals always have the same color, because of the presence of certain elements in the mineral's normal molecular structure.
- Gold is yellow because the element Au absorbs all other colors except yellow.
- Azurite is blue because the copper in azurite molecules absorbs all other colors except blue.
- Crocoite is red-orange because the chromium in crocoite molecules absorbs all the other colors.
An atom can be visualized as a nucleus surrounded by electron shells. These shells represent the different energy states available to that atom's electrons. Incoming energy, including visible light, can be used to raise an electron from one shell up to a higher one, but only if its wavelength is exactly right. When it is, the energy of that wavelength is completely absorbed. Other wavelengths either pass right through or are reflected back (or scattered). The wavelength of light that is mostly strongly reflected or scattered is what gives an object its visible color, because this is the one that most clearly reaches our eyes. To take the example of an emerald, it absorbs primarily the violet and red wavelengths, reflecting the green, and so the emerald looks green.
The primary elements that absorb visible-spectrum color are the transition metals on the Periodic Table. The most familiar of the 38 transition metals are iron (red, green, yellow), cobalt (blue), and nickel (green). Others include vanadium (red, orange), manganese (red), chromium (red, green), gold (yellow), titanium (blue), and copper (green, blue). Some rare earth elements also create color through their absorbtion patterns, including scandium, yttrium, lanthanum, and cerium.
Other minerals get their colors from elements that are not ordinarily part of their chemical makeup. To greatly simplify this exceptionally complex subject, here are the 4 most widely agreed upon causes of color in allochromatic minerals:
- Charge Transfer
- Color centers
Tiny amounts - as little as one tenth of 1% - of an impurity in the molecular structure of a mineral can determine that mineral's color. The amount and type of impurities affects the color of the mineral.
- Amethyst: Add some iron to clear, pure quartz and it becomes purple.
- Citrine: Inclusions of Fe+++ cause the yellow color in citrine.
- Rose Quartz: Trace amounts of titanium or manganese turn quartz pink.
- Green fluorite: X-Ray diffraction tests of fluorite from the William Wise Mine in New Hampshire showed that the element yttrium, a known coloring agent, constituted 0.2% of the sample.
- Aquamarine: Inclusions of iron (Fe++) are responsible for the blue color in beryl.
- Heliodor: Iron (Fe+++) in beryl's molecular structure turns it yellow.
- Morganite: Manganese as Mn++ turns morganite pink.
- Red beryl: The red color is caused by Manganese (Mn+++).
Inclusions of a second mineral (or even air!) within a host mineral can also alter a mineral's color.
- Prase quartz: Hedenbergite inclusions in quartz give prase its green color.
- Green quartz: Chlorite inclusions in quartz also cause it to turn green.
- Jasper: Hematite inclusions can turn jasper red.
- Milky quartz: Inclusions of very small air bubbles in clear quartz turn it white, in which case it is called milky quartz.
- Rutilated Quartz: Rutile inclusions in quartz give it a golden hue.
3. Charge Transfer
When two or more elements in a mineral exchange electrons, this is called charge transfer. The movement of electrons results in selective absorption of light.
- Sapphire: Sapphires contain small amounts of titanium and iron. The electron transfer between Ti and Fe causes light in the yellow through red spectrum to be absorbed, producing the deep blue color sapphires.
- Aquamarine: Small amounts of iron in valence states Fe++ and Fe+++ cause an electron transfer that absorbs red light, resulting in the color blue.
- Tourmaline: When manganese (Mn++) and titanium (Ti4+) swap electrons, it creates a yellow-green color.
4. Color Centers
A color center is a defect in the molecular structure of a mineral. The defect is often due to damage from heating or natural radiation. UV light (from prolonged exposure to sunlight, for example) can also cause color change. Color centers can be removed by adding energy, as is sometimes done by heating gemstones to change their color. There are two types of color center defects:
- An unattached excess electron can be trapped in a void in the mineral's structure.
- A missing electron causes a hole in the molecular structure.
In both cases, the flaws result in the absorption of certain wavelengths of light, and the colors not absorbed are seen as the mineral's color.
- Purple fluorite: When a negative ion is missing from the structure (sometimes referred to as a Frenkel defect), it creates a site that attracts and traps a free electron The trapped electron creates an electron color center, which is responsible for the color in purple fluorite.
- Smoky quartz: Radiation, either natural or man made, creates a hole in the silicon dioxide molecule, which changes colorless quartz to smoky quartz.
- Topaz: The color in natural yellow, orange or brown topaz results from a color center that is stable to light. If colorless topaz is irradiated, the radiation creates a hole in the color center, and changes the clear color to yellow, orange, brown or blue.
Written by Eric Greene and Helene Grogan