How to Test Crystals with a Black Light

Many crystals exhibit fluorescence when exposed to ultraviolet radiation from a source like a black light. Testing crystals with a black light, in combination with other tests, can help determine the identity of the crystal. Some diamonds exhibit blue fluorescence, while fake diamonds rarely fluoresce. Artificial sapphires are doped with fluorescent compounds so they can be distinguished from natural sapphires. Different minerals fluoresce in a rainbow of colors that can help you identify them under a black light.

Things You'll Need

• Black light
• Dark viewing area
• Crystals

Instructions

1. 

   Different mineral crystals fluoresce in a wide range of colors, depending on their composition.

   Set up the black light. The light should be in a dark viewing area. The UV (ultraviolet) radiation from a black light is harmful to your eyes and you should not look directly at a black light. Shield the light so it shines only on the crystal.

2. Turn on the black light. With normal room lights still on, turn on the black light.

3. 

   Artificial sapphires are doped so they fluoresce.

   Place the crystal under the black light.

4. Turn off the normal room lights and observe any fluorescence from the crystal. It may take a few seconds for your eyes to adjust to the dark.

5. Note the color of the fluorescence. Turn the room lights back on and turn the black light off.

6. Assess the fluorescence. Different mineral crystals fluoresce light in white, red, orange, yellow, green or blue. Use shortwave or longwave black lights for a more discerning test. See Resource 1 for a list of minerals and the colors they fluoresce under different black light radiation.

References

• National Jewelers Supplies: Diamonds--Real or Fake

Resources

• Galleries: Mineral Gallery--The Fluorescent Mineral Table