Ultraviolet Fluorescence

Some rocks and minerals have a hidden side that only appears when the lights go out.

When exposed to ultraviolet (UV) light, certain materials absorb energy and immediately release some of it as visible light. This phenomenon is called fluorescence.

The result can be dramatic. A specimen that looks ordinary in daylight might suddenly glow bright green, orange, red, blue, yellow, or another color under ultraviolet light.

Ultraviolet Fluorescence — Learning Series 002 (PDF)

Why Do Some Minerals Fluoresce?

Fluorescence usually occurs because of small amounts of particular elements or structural features within a mineral. These are sometimes called activators.

Interestingly, impurities can also prevent or reduce fluorescence. Two specimens of the same mineral may therefore react very differently under the same UV light.

That is one reason fluorescence is so fascinating—and why a UV response alone should not normally be considered proof of a mineral's identity.

Longwave and Shortwave UV

Not all ultraviolet light is the same.

Mineral collectors commonly use different UV wavelengths, particularly longwave (LW) and shortwave (SW) ultraviolet light. A mineral may fluoresce under one wavelength but show little or no reaction under another. Some materials respond to both, sometimes with different intensity or even different colors.

The UV flashlight you already own may therefore reveal only part of the story.

More Than Just a Pretty Glow

Ultraviolet light can be a useful tool for rockhounds, mineral collectors, gemologists, and lapidaries.

Fluorescence can sometimes provide an additional clue when identifying a material. UV examination may also reveal differences within a specimen that are difficult to see in normal lighting, and in some cases it can help identify treatments, fillers, adhesives, coatings, or other materials.

But UV should be treated as one piece of evidence, not a magic identification test.

Color, hardness, crystal form, specific gravity, magnification, provenance, and other observations may all be needed to reach a reliable identification.

Fluorescence, Phosphorescence and Other Reactions

Fluorescence occurs while the material is being exposed to ultraviolet light.

Some materials continue to glow after the UV source is removed. That phenomenon is called phosphorescence.

These effects are part of a much larger and fascinating subject involving the interaction of minerals and light.

Start Here: Ultraviolet Fluorescence

Want to explore the subject further?

Start with Learning Series 002 — Ultraviolet Fluorescence, an introduction to why minerals fluoresce, UV wavelengths, fluorescent colors, and how ultraviolet light can be used when examining rocks, minerals, and gemstones.

Ultraviolet Fluorescence — Learning Series 002 (PDF)

More UV Topics Are Coming

As the Learning Center grows, this section may include additional examples of fluorescent minerals, comparisons under different UV wavelengths, practical tips for examining specimens, and examples of where UV can—and cannot—help with identification.

Sometimes the most interesting thing about a rock is something you can't see until you turn out the lights.