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There are a lot of great rocks out there, and we’ve even talked about some of them here on SciShow before.
But in our opinion, some of the coolest rocks are the ones that glow.
Any rock will glow if you heat it up enough, but there are a few types that are especially
good at giving off light, and they do it in all different ways.
Here are five of our favorite rocks — and how they get their glow.
The most famous glowing rock might be fluorite,
which is actually where the word fluorescence comes from.
When this mineral is exposed to ultraviolet, or UV light, it gives off an eerie blue, like
your shoelaces under a blacklight.
Essentially, it’s absorbing the UV light and then re-emitting it at a different frequency.
But fluorite and other rocks that fluoresce tend to only absorb and emit specific colors of light.
Shine the wrong color on them, and they just look like an ordinary rock.
And that’s actually a big hint about what gives them their glow.
Anything that’s sensitive to certain frequencies of light has probably
got something quantum mechanics-y going on.
See, in every material, electrons are bound to atoms in certain energy levels.
And for an electron to step up or down from one energy level to another, it has to gain
or lose a specific amount of energy.
Absorbing light is one way electrons gain energy.
But different colors of light have different amounts of energy, so electrons can only absorb
light with the exact right amount of energy to take them up to a higher energy level.
Once they get there, though, electrons don’t typically stay there.
They tend to lose energy and fall back to a lower energy state.
And as they fall back down, they often emit photons of light.
For fluorescent materials, you only need to wait a nanosecond or so to see that happen.
But the light they emit doesn't necessarily have the same energy as the light they absorbed.
Materials like rocks have lots of energy levels in them, so an electron might lose a little
energy to heat and drop down one level, and then drop down
the rest of the way by emitting a photon.
So the photons they emit each have lower energies than the ones they originally absorbed.
That’s why you can shine UV light on fluorite, but then see it glow in blue.
And because UV light is invisible to humans, it looks to us like fluorite glows in the “dark.”
Overall, though, fluorescence is actually pretty common: There are tons of fluorescent
rocks out there, and they glow in all different colors.
And what’s amazing is they just look like ordinary rocks until you see them under the
right kind of light.
Another mechanism that makes rocks glow is called phosphorescence, and it was also named
after a natural substance: phosphorus.
Except, confusingly, while phosphorus does glow, we now know it’s actually not from
phosphorescence — it’s from chemical reactions with the air.
So that’s another story.
But lots of other rocks do truly phosphoresce, like calcite.
Just like fluorite, calcite and other phosphorescent rocks glow under UV light.
But unlike fluorite, these rocks can truly glow in the dark.
Because even after you turn off the light, they’ll keep glowing for a while.
The reason for this is that phosphorescence works almost exactly like fluorescence, but
with one key difference.
In phosphorescent rocks, electrons that have jumped up to a higher energy level don’t
fall down in a split-second the way they do in rocks like fluorite.
Their structure lets them hold on to that energy for a lot longer.
That’s because, before they drop down to where they started, they usually have to go
through what’s known as a forbidden transition.
A forbidden transition is one that's just very unlikely to happen.
And it’s unlikely because it requires a change in the electron’s spin—that weird,
quantum mechanical property all particles have that’s vaguely like ordinary rotation.
And the thing is, transitions that involve a change in spin are less likely than ones that don’t.
Which means you have to wait longer to see it happen.
As a result, a phosphorescent rock that’s been charged up will slowly release that pent-up
energy as light over time, so you can see it glow for minutes or even days since it
was last exposed to light.
Phosphorescence in nature is actually fairly rare, because it’s hard to get a rock with
forbidden transitions at just the right energies needed to emit visible light in that way.
But if you ever had glow-in-the-dark stars on your ceiling at night, you’ve seen phosphorescence at work.
Now, if you’ve tried shining a UV light on a rock but you’re not having any luck,
you’re not out of options.
You can always try hitting it with something!
That’s the essence of triboluminescence, which is the type of glow you get when you
hit a rock hard enough—or rub it, pull it apart, or generally apply mechanical stress
to it in some way.
The term covers a lot of bases.
Scientists actually don’t understand triboluminescence perfectly, and it may work differently for different materials.
But we have a decent idea of how it works for rocks like quartz that give off light
when you rub them together.
As the rocks scrape against each other, some charged atoms and electrons on the surface get knocked free.
Then they collide with molecules in the air and knock loose even more charged particles.
As they recombine, those charged particles create little electric shocks, like mini-lightning,
that we see as flashes of light.
But this doesn’t just happen with any rock.
To get triboluminescence, scientists think you need a rock whose crystal lattice structure
is asymmetric in some way—in other words, it’s not just made of nice, neat rows of atoms.
That’s how some parts of the material end up with excess electric charge, which can
break free and produce that light when the material is put under stress.
Unlike fluorescence and phosphorescence, triboluminescence tends to be a brief flash of light.
And it’s a very common phenomenon—it’s possible that around half of all inorganic
compounds triboluminesce in some way, including rocks like quartz and feldspar, as well as
some everyday organic compounds like sugar.
Today, scientists are still studying this phenomenon and learning more about how it works.
Which is pretty cool—because it’s not often you can do science just by
smashing rocks together!
Now, as we mentioned at the start of this episode, one way to make a rock glow is by
heating it up.
But heated rocks can actually glow for more than one reason.
Things like lava or hot coals glow because of what’s called blackbody radiation: Heat
makes their molecules vibrate, and that releases energy in the form of light.
But there’s another, slightly safer way to get light from
a rock by heating it, called thermoluminescence.
This happens specifically when an object is heated after absorbing energy in some other way.
That energy usually comes from some kind of radiation, like X-rays, but light, friction,
and even pressure can sometimes do the trick, too.
Any of these processes can free electrons from the crystal lattice that makes up a rock.
But in thermoluminescent rocks, those freed electrons quickly get re-captured in imperfections in the lattice.
The thing is, these new bonds are weaker than the original ones, meaning the electrons actually
become easier to free again.
So a little heat is enough to vibrate the atoms making up the rock and set them loose.
This time, the electrons fall back down into their original positions.
Once again, like in fluorescent rocks, those electrons give off photons as they drop down.
The amount of light these rocks give off depends on how long they were exposed to the initial
source of energy.
The more electrons there are trapped in different imperfections, the more light the rock will
release when you heat it.
And it takes only a little bit of heat to release a lot of stored energy, so it’s
actually pretty easy to get a bright glow out of a thermoluminescent rock.
For example, a rock called chlorophane will visibly glow just from the heat of your hand.
So if you pick up a rock and it starts glowing, it probably doesn’t mean you’re the chosen
one destined to save the world…
Probably.
Finally, if heating, lighting, and hitting a rock still won’t make it glow, you can
always try passing an electric current through it.
If it glows, that means your rock is electroluminescent.
Just like heating or shining light on a rock, passing an electric current through it is
a pretty good way to charge up its atoms with energy, as long as it conducts electricity
in the right way.
In electroluminescent rocks, the electric energy is enough to free electrons from their
atoms, leaving behind a bunch of positively-charged atoms and negatively-charged free electrons.
Since positive and negative attract, those particles will quickly recombine and become
neutral—and once again, you get that mini-lightning, that quick release of light.
The molecular structure needs to be just right, so this doesn’t happen very often in naturally
occurring rocks, but some colored diamonds are an exception.
See, in certain cases, diamonds aren’t actually completely transparent.
They can have flaws in their structure that make them white or pink, and they can also
have trace amounts of stray elements that make them other colors, like yellow or blue.
And those impurities are the key: Those little interruptions in the crystal lattice are what
emit light when the electric current passes through them.
What’s pretty wild about this is that the electroluminescence of diamonds could potentially
make them useful for quantum computing.
That’s because quantum computing relies on super precise control over individual particles.
And a well-made artificial diamond with a specific impurity can be sensitive enough
to emit one photon at a time through electroluminescence, which could help us
build more robust quantum computers.
So there are lots of glowing rocks out there, and lots of weird ways that they glow.
We actually didn’t get around to talking about a few other neat examples, like how
the radioactive element actinium glows as its radiation reacts with the air around it.
But y’know, there are plenty of safe ways to get glowing rocks, so maybe leave actinium
to the experts.
Thanks for watching this episode of SciShow!
And to find out even more about some of the coolest rocks out there, you can check out
our video on color-changing minerals right after this.
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