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.

Lessons with a Wagtail (Motacilla cinerea)

A Grey Wagtail (Motacilla cinerea) encourages fledglings to fend for themselves, by catching insects on the river.

That afternoon I’d found myself lying on the shingle bank of the river, with lens & camera resting on a stone just an inch above the water. It was a hot humid day, the morning had been busy and the river was enticing. I’d taken the 700 mm fl setup with me in the hope of spotting a Dipper whilst also cooling off my core temperature. I’d seen the Dipper.. on the wing at high speed – in no mood to pose & ‘dip’ for me. But when, some 30 minutes later, the Grey Wagtail appeared, I was glad that I’d been patient despite the insect harassment & the pebbles jabbing at my ribs. She stood just 4 or 5 metres in front of me, her attention not on feeding, nor upon the strange camo clad human lying in the damp shade..

A grey Wagtail (Motacilla cinerea) standing amongst stones on the river margins.
A Grey Wagtail (Motacilla cinerea) standing amongst stones on the river margins.

Her attention was focused a little upstream (behind me of course!) where young fledglings were learning to catch insects for themselves. A Minnow rose to take a mayfly from the river surface just to my right, the adult Wagtail flew upstream & I realised that I would need to move & reset if I was to record any of this story.

Grey Wagtails are insect eating birds. In summer they are mainly to be found along fast flowing streams & rivers where food is plentiful. Chicks fledge after only 2 weeks or so. In the UK their conservation status is considered red, so it’s pleasing to see them breeding successfully.

A young Grey Wagtail ponders how to catch a snack for himself.
A young Grey Wagtail ponders how to catch a snack for himself.

Some time later I was re-positioned partially submerged in the rapids by some boulders and the Wagtails were back. I spotted two fledglings, ok for a later brood, though 3 would have been nice. They hadn’t fully mastered the skill of catching dinner, so frustrating when it’s hovering just inches away – as in the picture above.

The adult demonstrated the technique necessary by deftly plucking an invertebrate from the fast shallow flow at the top of a little cascade.

With skill & practice, a Grey Wagtail pecks insects out of the fast flowing shallows.
With skill & practice, a Grey Wagtail pecks insects out of the fast flowing shallows.

The lessons must have been tiring, one youngster decided to take a break; fluffing himself up and looking all cute in the afternoon sunshine.

A Grey Wagtail (Motacilla cinerea) fledgling stands on riverside rocks watching its parent search for insect food.
A Grey Wagtail (Motacilla cinerea) fledgling stands on riverside rocks watching its parent search for insect food.

If you’re out walking the river banks this summer, keep an eye open for these delightful little birds.

Redstart Family Fledging

Most years we notice Redstarts arriving in spring and successfully raising a family. This year I have been particularly happy because a pair chose to nest in the double hedgerow that I renovated & planted a few years ago. This has meant that we’ve had the pleasure of watching at least 3 youngsters fledge; quite literally flitting across the hedged walkway that we created. The featured title image above shows the male calling out the fledglings with one vociferous fledgling close by.

Redstarts (Phoenicurus phoenicurus) are members of the Family Muscicapidae or Old World Flycatchers, a group that is well represented locally. In summer the male has quite striking plumage as shown below:

A male Redstart (Phoenicurus phoenicurus) calling to encourage his fledglings to explore.
A male Redstart (Phoenicurus phoenicurus) calling to encourage his fledglings to explore.

They arrive in the UK during April with the males often being a few days in advance of the females. Once paired up, about 5 or 6 blue eggs are laid in a nest. It will take 2 weeks for them to hatch & another 2 to 3 weeks before they fledge.

The female does not have quite such a bright plumage as the male, but she can be seen to flick her warm coloured tail on a regular basis:

Female Redstart (Phoenicurus phoenicurus)
Female Redstart (Phoenicurus phoenicurus)

After the parental collection of many spiders, flies & worms (with maybe a few berries too), the young chicks will have grown sufficiently to fledge. The male has certainly been giving encouragement and guidance to the nestlings that we’ve seen. Sitting very quietly in long grass at the side of our walkway, I watched as first one & then all three tried a first short flight of about 7 or 8 feet across to some young hedgerow trees. Here’s a picture of one of the fledglings perched in his new found tree:

A young fledgling Redstart (Phoenicurus phoenicurus) takes perches in a hedgerow tree.
A young fledgling Redstart (Phoenicurus phoenicurus) perches in a hedgerow tree.

Were there more than 3 fledglings? Possibly, I couldn’t clearly see the nest exit, there was definitely much chirping! Will this year be good enough for these little amber listed passerines to attempt a second brood before leaving for Africa this autumn? They should have the time available, I shall certainly keep a watch. It’s very pleasing to know that our hard work does seem to be helping at least some of the wildlife.

Crafnant and Geirionydd – twin lake walk

This walk is a very pleasant 5 mile walk with about 1200 feet of climb. Parts of it are along well made stone track, some is rougher going with many roots under foot. We walked it early one summer’s morning during a heatwave.

Early morning light casts reflections in beautiful Llyn Crafnant
Early morning light casts reflections in beautiful Llyn Crafnant

To locate the car parking, travel north to Trefriw from, Betws-y-Coed. As the village road crosses the river with the woollen mill on your left, turn left & steep uphill at the side of the mill. Carefully follow this very narrow lane and its signs for Llyn Crafnant. Just before you reach the lake, there is a forest car park on the right (free at the time of writing). Having parked up, walk a little further up the lane, turning right at the beginning of the lake and following the path on its north-western side. There are beautiful views & many reflections to be seen as you walk along this lakeside path.

Picturesque and Idyllic
Picturesque and Idyllic

Crafnant Pathway
Crafnant Pathway

Be careful to keep to the main lake side path, not straying uphill & away from the lake on any branched tracks. Towards the head of the lake, the well sign-posted path will take you through several gateways & around on to the other bank; follow the small lane for a little way until an obvious footpath strikes off to your right, uphill & into woodland. Take this path (as per the route map below) and follow it over the wooded hillside.

Trail of light and dark
Trail of light and dark

As one descends in to the adjacent valley floor, you will emerge from the woodland at the head of Llyn Geirionydd. Cross a style to stay on the near side of the lake (rather than crossing to the lane on the eastern bank). Whilst this path is a little rough in places, it is easy navigation, just follow the lakeside.

Llyn Geirionydd
Llyn Geirionydd

This is a peaceful place when water-sports are not taking place on the lake and there is plenty of wildlife to look out for. We were serenaded by the morning calls of a Sandpiper and were lucky enough to capture a bit of video footage:

Sandpiper Calling from AnnMarie Jones on Vimeo.

A Sandpiper calling out from a branch overhanging Llyn Geirionydd.

At the north end of the lake, keep straight ahead to view the Taliesin monument. This commemorating the reputed birthplace of Taliesin, chief bard of the 6th century. Whilst here Tamsin heard a crunching sound, which turned out to be a Golden-ringed Dragonfly having some breakfast (see Gallery at end for a photo).

Taliesin Info Plate
Taliesin Info Plate

Do look back from the monument at the view back along the length of the lake.

Monument on the shores of Llyn Geirionydd commemorating the birthplace of Taliesin, chief bard of the 6th century.
Monument on the shores of Llyn Geirionydd commemorating the birthplace of Taliesin, chief bard of the 6th century.

Once done here, continue roughly northwards along the clearly defined & waymarked path until it crosses a stonewall via ladder-style. At this point be sure to take the path ahead & uphill. We will now pass back over the hillside in our return towards the car park. On the way back down, keep an eye out for the old quarry workings.

Looking for Quarries
Looking for Quarries

The cool air emanating from a little mining tunnel was very enticing on such a warm morning and of course Tamsin couldn’t resist exploring.

Indiana Tamsin
Indiana Tamsin

Meanwhile I walked over the top of the spoil, only for us both to discover the same quarried cave. The dripping water, cool shade, coloured rocks and imagination inspiring mouth; made this an interesting bonus to the end of our walk.

Quarry Opening
Quarry Opening

From here it is but a 5 minute walk down a forest track and back to the car. An enjoyable 3 hour walk on a glorious morning.

GPS Route:

Photo Gallery:

Machynlleth and Llyn Glanmerin – a short walk

A short (~ 4miles with 750ft climb) circular route from Machynlleth to visit beautiful Llyn Glanmerin. This is a relatively easy walk for any regular walker, including children. It does have a climb near to the start, so some fitness is advised. Tamsin and I walked this at the beginning of June whilst our car was in the garage for maintenance. There’s plenty of views & wildlife to take in on this pleasant stroll.

For convenience I have started the walk from the main Pay & Display Car Park in Machynlleth (Loos available at CP entrance). Exit the back of the car park and turn left on to a pleasant tarmac walkway which will lead you past the library and on to the high street. Turn right & follow the pavement until you reach the road turning right for Forge & Dylife, take this turning. Close to the end of the housing you will see a signposted footpath leading to the right. Follow this footpath away from Machynlleth, uphill on the right edge of the local common land. As you climb above the golf course on to bracken & heath, do look back at the view of Machynlleth nestled amongst the hills of Dyfi.

Machynlleth Panoramic
Machynlleth Panoramic

Continue uphill across this now heathy common land. The track now strays from the righthand edge a little, crossing open land from one block of woodland edge to another. Keep an eye open for Red Kites gliding across the skies above you.

Red Kite against blue skies
Red Kite against blue skies

When you meet Glyndwr’s Way crossing you from wooded left to open heath right, keep straight ahead into a narrow wooded path that takes you on a brief detour to Llyn Glanmerin. As you emerge from the forestry Llyn Glanmerin is immediately down to your left. Llyn Glanmerin is sometimes referred to as Lord Herbert’s lake. Lord Herbert Vane Tempest of Plas Machynlleth was a director of the Cambrian Railways. He sadly died, along with 16 others, in the Abermule train crash of 1921. The lake itself is of about 7 acres in area and is most picturesque with water lilies floating upon it and Rhododendrons on its banks.

Banks of Llyn Glanmerin
Banks of Llyn Glanmerin

Water Lily on Llyn Glanmerin
Water Lily on Llyn Glanmerin

On a warm summer day Dragonflies & Damselflies are to be seen patrolling the water’s margins in search of a mate or laying eggs beneath the surface.

Skimmer Dragonfly
I believe this to be a female Black-tailed Skimmer (Orthetrum cancellatum) Dragonfly; with dark pterostigma yet golden costa.

Now briefly retrace your steps through the narrow section of woodland, back to Glyndwr’s way. Upon meeting the Glyndwr’s way path, turn west along it and over the open heath land. This well marked path will then lead you downhill towards the Cae-Gybi lane. As you descend, see if you can spot Plas Machynlleth (Lord Herbert’s old home) beneath you at the edge of Machynlleth.

Plas Machynlleth
Plas Machynlleth

Upon meeting the lane, turn right along it briefly, before again leaving it to follow Glyndwr’s way down to Plas Machynlleth. Turn right & walk along the path through Plas Machynlleth’s grounds. This will lead you to the back of the car park from whence you began the walk.

Route Map:

Gallery: