A gem's color comes from the light it absorbs. White light contains every color, and when a stone absorbs some wavelengths, the ones that pass through or bounce back reach your eye. A ruby looks red because it absorbs much of the yellow-green and violet light and lets red through. What does the absorbing varies: elements in the crystal, gaps in its structure, or the way its internal structure bends and splits light.
Idiochromatic and allochromatic gems
Gemologists split colored gems into two groups by where their color comes from.
Idiochromatic gems are "self-colored." The element that causes their color is part of their chemical formula, so every specimen has a version of that color. Peridot is green because iron is one of its main ingredients. Malachite and turquoise owe their green and blue to copper in their formulas. Spessartite garnet is orange because of manganese. Idiochromatic gems tend to vary within a narrow color range.
Allochromatic gems are "other-colored." In pure form they are colorless, and their color comes from trace elements or defects that are not part of the main formula. Pure corundum is colorless. Add a little chromium and it becomes ruby; add iron and titanium and it becomes blue sapphire. Pure beryl is colorless goshenite. Chromium or vanadium makes emerald, iron makes aquamarine, and manganese makes morganite. Allochromatic species can come in a wide range of colors.
Some gems fall in a third group, pseudochromatic, where color comes from the physical structure of the stone instead of chemistry. Opal's play of color is the classic case.
Chromophores
The elements that cause color are called chromophores. Most are transition metals, whose outer electrons absorb specific wavelengths of visible light. The same element can produce different colors depending on the crystal it sits in.
| Chromophore | Gems it colors |
|---|---|
| Chromium | Ruby, emerald, red spinel, alexandrite, tsavorite, chrome diopside, pink topaz |
| Vanadium | Some emerald, some tsavorite, color-change sapphire |
| Iron | Peridot, aquamarine, almandine garnet, citrine, amethyst (with irradiation), green sapphire |
| Iron and titanium together | Blue sapphire |
| Copper | Turquoise, malachite, Paraiba tourmaline, Oregon sunstone |
| Manganese | Morganite, rhodochrosite, spessartite, kunzite, pink tourmaline |
| Cobalt | Some blue spinel, cobalt glass |
| Nickel | Chrysoprase |
Chromium shows how much the host crystal matters. In corundum it makes red. In beryl, a slightly different setting around the chromium atom shifts its absorption, and the result is green. In chrysoberyl, the setting falls in between, so alexandrite looks green in daylight and red in incandescent light.
In blue sapphire, iron and titanium sit next to each other in the crystal and an electron moves between them when light hits. This process, called charge transfer, absorbs strongly and gives intense color even from tiny amounts of the elements. The sapphire profile covers its many colors.
Color centers
Some gem colors come from defects in the crystal structure rather than from chromophores. A missing atom, an extra electron or a displaced ion can trap light energy and absorb certain wavelengths. These are color centers.
Natural radiation in the earth creates color centers over long periods. Smoky quartz gets its brown color this way, and amethyst's purple depends on iron combined with natural irradiation. Many green diamonds owe their color to natural radiation. Labs can create color centers with irradiation: most blue topaz on the market is colorless topaz treated this way and then heated.
Color centers can be unstable. Heat and sometimes strong light can undo them. That is why heating amethyst can turn it yellow or orange (much commercial citrine is made this way) and why some irradiated colors fade.
Color from structure
Some gems produce color through physical optics.
- Play of color in opal comes from orderly stacks of tiny silica spheres. When the spheres are the right size and evenly arranged, they diffract white light into spectral colors that shift as the stone moves.
- Labradorescence in labradorite comes from thin, alternating layers within the feldspar that cause interference colors.
- Iridescence in pearls and some fire agate comes from thin layers that interfere with light, much like a soap bubble.
These effects are covered with other optical phenomena on the gemstone optics page.
Color change and pleochroism
A few gems change color with the light source. Alexandrite is the best known: green to bluish green in daylight or fluorescent light, red to purplish red under incandescent light or candlelight. Some sapphire, garnet and spinel also change color. This happens because the gem transmits two bands of color about equally, and the light source tips the balance.
Pleochroism is a different effect. It is a change in color with viewing direction, caused by double refraction in the crystal. The pleochroism page explains it.
Hue, tone and saturation
Gemologists describe color with three components.
- Hue is the color family: red, orange, yellow, green, blue, violet, purple, or a mix such as purplish red or greenish blue. Most gems show a main hue with a modifier.
- Tone is how light or dark the color is, from very light to very dark.
- Saturation is the strength or purity of the color. Low saturation looks grayish or brownish; high saturation looks vivid.
The most desirable colors in most gems have a pure hue, medium to medium-dark tone and strong to vivid saturation. A stone too dark loses brightness, and one too light looks washed out. GIA's colored stone grading system uses these three terms, and the colored gemstone grading page goes into how they affect value.
Lighting changes how color looks. Daylight, fluorescent light, LEDs and incandescent bulbs each bring out different hues. Judge a colored gem under more than one light before you buy it.
Treatments that change color
Many treatments target color. Heating can deepen or lighten color, remove unwanted tints, or change it entirely. Irradiation creates color centers. Diffusion adds chromophores from outside the stone, such as beryllium in sapphire. Dyes add color to porous materials. These processes are covered on the gemstone treatments page.
Frequently asked questions
Why are rubies red and emeralds green if both get their color from chromium?
The chromium sits in a different crystal setting in each mineral. That shifts the wavelengths it absorbs, so it produces red in corundum and green in beryl.
What does idiochromatic mean?
It means a gem's color comes from an element that is part of its chemical formula. Peridot, turquoise and malachite are idiochromatic.
Can a gem's color fade?
Some can. Gems colored by unstable color centers, such as some kunzite, irradiated stones and some amethyst, can fade with heat or long exposure to light.
What is the most important part of color in valuing a gem?
For most colored gems, saturation and a pure hue matter most, along with a medium to medium-dark tone. Too dark or too light usually lowers value.