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How It Works
Black lights emit light in the ultraviolet (UV) spectrum, using electromagnetic radiation that reacts with the electromagnetic radiation in certain minerals. This causes the absorption and appearance of different wavelengths of light, which is how humans perceive color. You can use black lights to display your rock collection, but because not all rocks with fluoresce (glow) under the same wavelength, it is best to take a portable black light with you when you go to collect rocks. This way, rocks can be tested in the field and make sure they will enhance your collection.
Short Wavelength
Short wavelength black lights are the best choice for most rock and gemstone collectors. Also known as UV-C, these black lights emit wavelengths of light in the 200 to 280 nm (nanometer) range. This range reacts with the broadest selection of minerals and can cause your rocks to shine in a rainbow of colors. UV-C lights are also the most expensive type of black light on the market, according to the Fluorescent Mineral Society, ranging from $200 for a small, portable fixture to over $1,200 for more powerful models as of April 2010. You can find them at rock collector specialty shops (lapidaries) or order them online.
Medium Wavelength
Medium wavelength black lights, also known as UV-B or midwave black lights, can also benefit rock collectors. The optimal frequency for a variety of minerals is around 311 nm, and the medium wavelength spectrum ranges from 280 to 315. These lights won't cause the same vibrant reactions with most minerals as shortwave black lights, but they can be significantly less expensive, with some models offered in the $50 to $250 range as of April 2010. UV-B light wavelengths are the same ultraviolet wavelengths that cause suntans and sunburns.
Long Wavelength
Longwave ultraviolet lights (UV-A) are the most commonly found black lights, readily and inexpensively available at home improvement and department stores for as little as $2 for a bulb that fits into a standard light fixture. These simple bulbs may be less effective than UV-A lights that come in compact fluorescent fixtures. Long wavelength UV lights use the 315 to 400 nm part of the color spectrum, but react with only about 15 percent of the minerals known to fluoresce.
Display Considerations
Normal daylight and household light bulbs interfere with your ability to see the reactions between ultraviolet light and minerals, so minimize natural and artificial light near your display. For best results, lay out your rock collection on a black background and use blackout curtains on windows in the room when you want to view the collection at its full brilliance. Turn off all lights other than your black light. Some UV lights, particularly less expensive longwave lights, may require a UV filter over the light to block excess visible spectrum light from escaping.