Visible light is made of all the colors of the rainbow, adding up to give the "white light". A color that is absorbed by a mineral is the color that is taken up by the mineral, while the other colors are transmitted. The color of the mineral will be of the colors that are transmitted by the mineral, as illustrated below.
This red gemstone absorbs all the colors of the rainbow except the red color, which is transmitted.
Color-changeA color-change gemstone refers to a gem that changes color when illuminated under different lighting conditions. The typical example is alexandrite (a color-change chrysoberyl). Lights never display a perfect uniform emission of the visible light: some wavelengths can be more transmitted than others. For example, the sun is rich in blue light (also true for fluorescent light), while incandescent light (candle light for example) emits more red. Alexandrite has two "transmission windows" (due to the absorption of the element chromium): one in the blue and the other one in the red. When subjected to fluorescent or sun light, the stone will therefore appear blue, while it will appear purplish-red under incandescent light (combination of red and blue colors). Other gemstones can present this effect, such as sapphire or garnet for example.
DispersionThe color of light depends upon its wavelength. Normal white light contains a mix of all visible wavelengths and includes red, orange, yellow, green, blue, and violet (from longer to shorter wavelength). When a beam of light enters a transparent solid at an angle, it is refracted (the angle of the beam is changed). Longer wavelengths of light are refracted more than shorter wavelengths, so the material separates white light into its component colors. This phenomenon is called dispersion. Minerals differ in their ability to create dispersion. Diamond produces strong dispersion, which is the reason that one is able to see distinct flashes of color in an otherwise colorless diamond gem. The dispersion defines how efficiently a material splits white light into a rainbow (like a prism, see figure below).
In a more scientifically way, it is a measure of the angular separation of refracted light of different wavelengths (specifically, blue light at 430.8 nm and red light at 686.7 nm) within a given material. Diamond has a fairly strong dispersion value of 0.044, while titanite (sphene) has a very strong dispersion value of 0.051.
FireThe fire of a gemstone refers to the flashes of color emitted by this gemstone. It depends on the dispersion, the cut angles, the lighting environment and the refractive index. For example, under same illumination and with a similar cut, titanite (sphene) will show more fire than diamond, which itself show much more fire than quartz.
Index of refractionWhen a beam of light strikes the surface of a transparent material at an angle, part will be reflected away and part will penetrate the material. The part of the beam that enters the material will be bent or refracted by an amount related both to the angle at which the beam strikes the material (the angle of incidence), to the density of the material, and to the light absorbing properties of the material. In general, the denser a material, the more the light entering it will be bent, but because additional factors affect the bending, this determination is not the same as a measurement of the density. Also the amount of bending may vary with wavelength of the light. By measuring the angles of incidence and refraction, a quantity called the index of refraction can be determined. This index can be used as an identifying characteristic for the material. At the contact of substances with different index of refraction, the light will be bent, with an angle defined by the relation: η1 sin(θ1) = η2 sin(θ2), where η1 and η2 are the index of refraction of 2 different materials, and θ1 and θ2 the angles of the propagating light, for material 1 and 2 respectively. By definition, the index of refraction of air is η = 1. The index of refraction of opal is 1.45 and of diamond is 2.435. Observe below the effect of the incident light on the refracted ray in these two different minerals.
Play-of-color.The play-of-color effect of opal is due to its internal structure: even if it is an amorphous material (no structure arrangement at the atomic level), there is an organization at the nanometer to micrometer level. Amorphous opal might present an organized network of spheres (such as the one shown below). It is the existence of this network of silica spheres that enables diffraction of the light, following Bragg's law: nλ = 2dηsinθ where n is the order of diffraction, λ is the wavelength of the incident light,d is the network spacing, η the index of refraction of opal, and θ the angle of the diffracted light with the network. n (=1), η (=1.45) and λ (=390 to 780 nm for the light the our eyes can see) are fixed, so only the size of the spheres and the angle of the incident light have an influence. That is why the color changes when moving a play-of-color opal (or when the observer moves relatively to the opal).
Scanning electron microscopy image of a play-of-color opal from Tecopa, California. The silica spheres, about 250 nm in diameter, are well packed, enable the diffraction of light on this perfect network. © Gaillou.
Pleochroism.Pleochroism is an optical property observed in certain minerals in which light is absorbed differently as it passes through the crystal in different directions. Differences in the internal arrangement of atoms in the crystal in different directions account for the differential light absorption. Three distinct colors (trichroism) or two distinct colors (dichroism) may be seen as a crystal is held in front of a light and turned. This zoisite crystal (a.k.a. "tanzanite") is a nice example of pleochroism.
This video shows the pleochroism (trichroism) of tanzanite. © Gaillou & Morgan.