Although glow-in-the-dark rocks have been studied since the 19th century and eagerly sought by black light-wielding rock hounds on the shores of the Great Lakes, the chemistry behind this natural phenomenon wasn’t fully understood for some time.
However, a recent study published in the Chemistry of Materials journal unravels the secrets of glowing rocks and could drive materials research to create a better synthetic material. These findings could help to improve safety aids, such as emergency signage that can light up without a power supply.
What Makes a Rock Glow?
First off, what makes a rock glow in the dark? The First Law of Thermodynamics gives a clue to this natural phenomenon. In a closed system, energy cannot be created or destroyed — it can only be changed. For example, running an electric current through the filament in an incandescent light bulb will heat it till it glows to produce light.
In the case of a glowing rock, no heat is involved and the natural phenomenon that is seen is described as cold light, or luminescence. According to The Gem Society, a lot of rocks and gemstones luminesce when activated under UV light. This feature is often used to distinguish between natural and lab-grown gems.
The rocks luminesce as electrons within them react to an energy change. Unlike bioluminescence, where light comes from a biological enzymatic reaction, rock luminescence is more like the aurora borealis phenomenon, where charged particles from the sun react with atoms in the atmosphere to activate their electrons. When gems glow under UV light, it is the light itself that activates elements within the rock, pushing the electrons into a higher energy state. When these activated electrons decay back to a lower energy level, they release photons as light.
Smashing Rocks for Science
Hackmanite, or tenebrescent sodalite, was first described in the 1800s as a rock that glows pink when broken or placed in the dark. It also gives off a characteristic glow when activated by UV light. Atlas Obscura describes how members of the rock hounding community take to the shores of the Great Lakes after dusk, armed with UV flashlights to search out syenite rocks rich in sodalite.
Collectors know that there is a spectrum among samples of the rock; there are some that glow and others that don’t, with the glow ranging from fiery orange to a soft pink. This luminescence also varies in how long it lasts.
As Science Alert describes, to find out why there is a difference in duration, an international team of researchers analyzed samples of hackmanite gathered from Canada, Greenland, Afghanistan and Pakistan. Using techniques such as spectrometry and X-ray diffraction, the team determined the composition of each sample to find out how each element contributed to the natural phenomenon.
Deliberate Impurities in Synthetic Materials
When the researchers compared the composition profiles for each of the hackmanite samples with luminescence, they found that the ratios of certain impurities in the rocks made a difference in how brightly they glowed. The ratios also explained how long the glow persisted after the rocks were activated.
From the analysis, they found that there were two pairs of elements that directly correlated with the persistence of luminescence. In terms of chemical composition, hackmanite samples with more titanium relative to iron and more potassium relative to sulfur luminesced brighter and for longer. The important factor was the relative concentrations between each element pair. The research team concluded that abundance of one of the pair could quench excitation in the other and thus diminish luminescence.
Taking a Cue From Nature
Materials research often takes cues from natural phenomena like glowing rocks. Creating a new synthetic material is often easier if there’s something in nature to copy. But first, you need to know what’s behind the magical glow.
For example, green fluorescent protein (GFP) is commonly used in lab studies to show activity within a cell. Since it can be switched on and off depending on certain intracellular activities, fluorescence microscopy will show not only that GFP is inside the cell but also where it is located. ScienceMag notes that studying new species of jellyfish is uncovering more powerful versions of GFP.
The hackmanite findings showed how a natural impurity can influence how brightly a rock glows and for how long. With this new knowledge, tweaking the composition of new synthetic materials could help to create brighter and longer lasting emergency signage, for example. As the research team notes, studying nature can help us improve the performance of our copies.