There are several ways that minerals can emit light, besides the light that is emitted from exposure to daylight or the light from normal light bulbs.
Some of these ways involve special lamps that emit non-visible ultraviolet light (at least not visible to humans).
The light from these ultraviolet lamps reacts with the chemicals of a mineral and causes the mineral to glow; this is called fluorescence.
If the mineral continues to glow after the light has been removed, this is called
Some minerals will glow when heated; this is called
And there are some minerals that will glow when they are struck or crushed; this is called
The fluorescent minerals are those that emit visible light when activated by invisible ultraviolet light (UV), X-rays and/or electron beams. Certain electrons in the mineral absorb the energy from these sources and jump to a higher energy state. The fluorescent light is emitted when those electrons drop down to a lower energy state and emit a light of their own. Although most collectors do not have access to X-ray or high energy electron emitters, they do have access to affordable ultraviolet lamps. The visible light emitted after being activated by UV light is sometimes very colorful and can often be very different from the normal color of the mineral. Collecting fluorescent minerals is a popular hobby and experienced collectors can use fluorescence for identification purposes. At night or in dark mines or caves, fluorescence can be used to find certain mineral deposits and is a viable prospecting technique.
Ultraviolet light is any light between 10 and 400 nanometers in wavelength - a huge range. It is called ultraviolet because these wavelengths are below violet (and thus invisible). There are several recognized subsets of ultraviolet light.
Ultraviolet A (or UVA or long-wave UV light) is commonly known as "black light" and most people are familiar with its effects of making white clothing glow in the dark. This is due to whitening chemicals in detergents. UVA spans the wavelengths from 400nm down to 315nm, and does not cause sunburn although it does damage collagen (aging the skin), and has been implicated as a contributor to cataracts. It destroys Vitamin A in the skin and generates free radicals that may be responsible for most of the damage it can cause. UVA comprises over 98% of the ultraviolet in sunlight at the Earth's surface. It is also responsible for the quick (short term) tan that results from the oxidation of melanin and its release from skin cells. Note that UVA is not blocked by most sunscreens, and is not measured as a factor in sunscreen SPF. See the table of longwave fluorescent minerals.
Ultraviolet B (or UVB or medium-wave UV) are the wavelengths between 315nm and 280nm. These wavelengths are responsible for most sunburns and skin cancer, as well as the highly beneficial effects of Vitamin D which the skin naturally produces in huge quantities during exposure to UVB. The skin also responds to UVB by producing more pigmented skin cells (giving the long-lasting tan that takes two days to generate), which is very effective at blocking ultraviolet light and preventing damage. Note that the response of most fluorescent minerals to UVB (medium-wave UV) is not described, although some "black lights" emit UVB as a component of broad-spectrum UV.
Ultraviolet C (or UVC or Short-wave UV light) spans the wavelength range from 280nm down to 100nm, although wavelengths below 200nm are strongly absorbed by the oxygen in air. Note that ordinary fluorescent lamps use mercury vapor which emits primarily at 254nm and the interior of the tube is coated with various minerals which fluoresce to generate the white (or other) light to be emitted. Simply omitting those phosphors and blocking longer wavelengths would result in a short-wave UV lamp. Shortwave lamps which are available to collectors can be very entertaining and useful to identify minerals, however it is dangerous to look at the shortwave light source (it can cause blindness) and they should not be used without adult supervision. See the table of shortwave fluorescent minerals. A lamp that can separately emit longwave and shortwave light is preferred as many fluorescent minerals emit different colors under different wave lengths and some only fluoresce under one but not the other.
Remember, the shorter the wavelength, the higher the energy per photon, and thus the more dangerous the radiation. UVA is less dangerous than UVB which is less dangerous than UVC. The still shorter wavelengths of ultraviolet light (VUV Vacuum UV, and EUV Extreme UV) are even more dangerous, but luckily they are absorbed by air and only pose a hazard during close exposure to a source such as an electric arc welder.
Activator elements are responsible for fluorescence in minerals. But not all specimens have these activator elements. Care in identifying minerals, using UV fluorescence, should therefore be taken. Some minerals will have consistent colors as in they will always fluoresce red for example while other minerals may have many different colors from one locality to another. Also, one specimen may fluoresce and another specimen of the same mineral may not fluoresce at all. So how can fluorescence help in identifying minerals?
Well, fluorescence is not a common phenomenon being found in only certain minerals. If two minerals are similar and yet one is listed as a possible fluorescent mineral, a fluorescence test could prove important. However if an unknown mineral does not fluoresce, it should not so quickly be dismissed as not being the suspected fluorescing mineral, unless that mineral is reported to always fluoresce. Fluorescence is not usually an absolute property found in all specimens of even a named fluorescent mineral, but there do exist some minerals that are so reliably fluorescent that fluorescence is the best test to use. The table below includes the more common fluorescent minerals that are popular with collectors. Compare the minerals found in different wavelengths and in different colors.