Fluorescence of rocks and minerals

Fluorescence, I’ve always found it fun, intriguing and often rather beautiful. I’d like to take my interest in fluorescence & combine that with my childhood love of collecting rocks (I so remember my Father’s warnings about not filling his car boot up with rocks, next time 😀 ). Sometimes the word fluorescent is misused to casually refer to an item that is extremely vivid & reflective; what is its scientific meaning?

When a substance absorbs incident electromagnet radiation (for example – light) and very shortly afterwards re-emits that absorbed energy as light of a longer wavelength, this is called fluorescence. Fluorescence can be useful for various applications – from medical research to identification of gems. Here we are going to investigate the natural fluorescence of minerals / rocks …

A rhomboid crystal of calcite illuminated by 'white' light (top) and by 254nm UV light (bottom).

Sample 1 – Calcite, a stable form of Calcium carbonate.

Calcite forms a range of beautiful crystals which can be variously coloured by impurities. When fairly pure & viewed in daylight (or with a studio lamp, as photographed here) it appears white, with a variable opacity.

However, if we exclude the daylight and illuminate the calcite with shortwave ultraviolet light (invisible to the human eye); then the calcite appears intensely red. This is due to absorbtion of the 254nm UV light and the subsequent fluorescent emmision of longer wavelength red light; which we and the camera can see.


How does fluorescence happen?

Think back to your chemistry lessons – imagine an atom or molecule. It’s happily sat there minding its own business, electrons busily orbiting the nucleus; when some shortwave UV-light shines upon it. Ping! One of its electrons gets all excited and jumps up to a higher excitation level. This isn’t very stable and so the electron must soon return to its normal ‘ground’ state. When it does so, some energy is released as light, and this we see as fluorescence.

For further info, Wikipedia has a more detailed article.


A group of botryoidal aragonite crystals illuminated by 'white' light (top) and by 254nm UV light (bottom).

Sample 2 – Aragonite, another form of Calcium carbonate.

Normally viewed as a creamy colour, this botryoidal crystal group emits a low blue-greenish fluorescence when viewed under UV-light.

Other aragonite crystals are known to emit a pinkish fluorescence.


A sample of Fluorite from county Durham, UK. Illuminated by 'white' light (top) and by 254nm UV light (bottom).

Sample 3 – Fluorite, Calcium fluoride.

Fluorite, the mineral that gives fluorescence its name. This predominately green sample is from County Durham, UK.

When viewed under the ultraviolet lamp, it fluoresces strongly with a deep blue-violet colour.


Ultraviolet Light – info & warning.

Violet is the shortest wavelength (highest frequency) light that is visible to the human eye. Beyond that lies the invisible ultraviolet, which is often sub-categorised as follows:

UV-A 315 nm to 400 nm example ‘Black lights’ used for effects shows

UV-B 280 nm to 315 nm

UV-C 100 nm to 315 nm example germicidal lamps

Some geological specimens will fluoresce at a wide range of UV wavelengths but most are best illuminated with shortwave UV-C lighting; like the 254 nm lamp I have been using. This comes with risks for the user and relevant protection should be maintained. Do not expose skin to the lamp’s light, do wear protective glasses and never look directly at the lamp.


Nodules of chalcedony illuminated by 'white' light (top) and by 254nm UV light (bottom).

Sample 4 – Chalcedony nodules

Chalcedony, a widely variable, mixed crystalline form of Silicon dioxide.

The rather dull nodules of this specimen transform to a mix of orange & violet when illuminated by shortwave UV-light.


Photographing the fluorescent samples.

Any standard camera should be suitable for photographing these subjects but the following features & accessories would be helpful:

  • macro or close focusing ability
  • long exposure options
  • remote or timed shutter release
  • tripod or other sturdy mount

Your needs will vary dependant upon the size of your rock samples and the strength of your lighting source. I chose to work with a relatively low power UV-C lamp, somewhat reducing cost & safety concerns but this does then necessitate long exposures.

I also built a blackout box to help with the photography. I used some 7 x 1 inch shelving timber to construct a small box with a 1/2 open front for the camera to look through. The inside is painted with matt black acrylic (thanks Tamsin) and the roof has a small slot for the UV-C lamp to rest in. A black gloss ceramic tile is used as a base, this gives a nice reflection when photographing with ‘white’ light. Finally, black material was draped over the entire setup, including the tripod mounted DSLR, thereby further reducing any extraneous light.