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When it comes to sparkly objects, the planet Earth has a lot to offer.
For examples of those, we usually think of minerals — solid structures made out of
repeating molecular patterns.
They’ve got a lot of cool properties, and if you put several minerals together, you
get a rock.
But there are also rocks that aren’t made of minerals.
They’re made of glass — that is, a solid that has a random, or amorphous, molecular
structure.
That doesn’t mean they all look like a window pane or a phone screen.
Nature makes all sorts of glass, mostly out of a molecule called silica, or silicon dioxide.
But the variety of forms it can take might surprise you.
Here are five examples of natural silica glasses with some pretty rockin’ origins.
Obsidian might be the most famous naturally-occuring glass.
Commonly referred to as volcanic glass, this dark, shiny rock is about 65 to 80% silica.
Most obsidian you’ll see is black, which is actually due to the presence of microscopic
crystalline impurities that make the stone opaque.
But the presence of various impurities can produce different colors.
Iron oxide can give you reds and browns, microscopic crystals of a mineral group called feldspar
can create a rainbow of colors, and trapped gas bubbles can produce a golden sheen.
You might even see larger mineral crystals trapped inside, creating a snowflake pattern.
It takes really thick volcanic magma to make obsidian.
That’s the stuff high in silica content.
Basically, the chemical bonds between silica molecules restrict movement, whereas if there
are metal ions present, they can break that structure and make the liquid rock flow faster.
So thicker magma equals more silica.
This silica-rich magma starts out with about 10% water content, but that water soon turns
to steam as the molten rock approaches the Earth’s surface and it’s no longer under
so much pressure.
That thickens up the magma even more, making it cool more quickly, and also stopping molecules
from arranging themselves into a crystalline structure.
So while obsidian can form above ground in a lava flow, most of it’s made underneath
the surface, cooling and solidifying before it ever gets up here.
Opals may be the most colorful of gemstones, and the most well-known variety owes its rainbow
flecks to its amorphous structure.
Like many glasses, opals are made of silica, but these stones also contain water.
Yes, water can be a component of rocks.
They’re made when water full of tiny silica particles, around 25 nanometers in diameter,
flows through and eventually gets stuck in cracks and pores in other types of rock.
Oftentimes, these silica particles grow like pearls, with shells of silica building up
layer upon layer.
This produces a wide range of sizes of silica spheres.
But if they’re similar enough in size, as they continue to get deposited by flowing
water, they settle into a series of horizontal, parallel planes.
But after all that you still don’t have an opal — later, another collection of silica
particles has to settle into the voids between all the spheres.
It acts as a cement, and traps a lot of water along with it.
But that silica doesn’t have a structured pattern to it, so our opal can’t be considered
a mineral.
It’s the spheres and the planes they lie in that give opals their optical properties.
In gemology, it’s called “play of color”.
Because of their different composition and structure, the silica spheres and the cement
around them bend light different amounts.
This creates what’s called a diffraction grating, and splits white light into its constituent
colors.
But just like waves on the surface of a pond can interact with and interfere with one another,
so do the light waves inside the opal.
Some colors get amplified, and others canceled out.
Which colors we ultimately see depends on how big the spheres are.
If they’re less than 138 nanometers in diameter, they only bend invisible ultraviolet light.
But once they hit that size, they produce violet.
And as they get bigger, the colors pass through blues and greens all the way to reds.
Red is actually pretty rare for opals, since it needs spheres over 241 nanometers in size.
But those large spheres also create all the other colors, which makes those opals the
most colorful, and the most highly valued.
If the spheres aren’t uniform in size — as little as 5% variation — they can’t stack
into parallel planes.
Light still gets bent, but the overall diffraction effect gets canceled out within the stone,
so no pretty colors reach our eyes.
Most gem-quality opals are mined in Australia, but Australia is also home to the only known
specimens of opalized fossils, from crustaceans to dinosaurs to early mammals.
And opals have even been found in at least one meteorite!
But speaking of meteorites, when they’re crashing into the Earth’s surface can make
glass.
In the modern day, they’re called tektites, from the Greek word for 'molten'.
However, the first written reference to them — from over a thousand years ago — was
Chinese for 'Inkstones of the Thundergods'.
Which obviously sounds way cooler.
Tektites come in sizes from tens of micrometers up to 10 centimeters long, and they’re formed
when a meteor hits sandy or rocky ground and the energy from that impact heats up and melts
that ground around it.
Molten blobs are thrown up into the air, then cool into glass on their way back down.
For a lot of the 20th century, scientists thought they had extraterrestrial origins
— that they were the glass version of meteorites.
One hypothesis in particular supposed that they had melted into glass on the Moon immediately
after an impact, been kicked off the surface with such force they escaped the Moon’s
gravity, and fell down to Earth.
But if that were true, they’d be distributed a lot more uniformly on Earth’s surface
than they really are.
Instead, they’re found in a few regions called strewnfields.
**That’s actually how types of tektites are named.
For example, Moldavite is named for the Moldau River in the Czech Republic, Australites come
from Australia, and Philippinites, well, you get the idea.
While there is quite a range, an average tektite is made up of around 70% silica, and has a
makeup somewhat similar to granite.
They come in shades of green to brown to black.
And they come in a variety of round-ish shapes, at least until erosion sets in.
Using our knowledge of radioactive decay, we can determine how old different tektites
are.
The oldest batch hails from Haiti and northeastern Mexico, dating back to the Cretaceous-Paleogene
transition — a geologic boundary coinciding with the extinction of the non-avian dinosaurs.
Which is also where you find the impact crater suspected to have been left by the meteorite
that did them in.
Meaning these tektites could be a 66 million-year-old relic of that very impact.
Another outdated hypothesis for tektite origins was lightning.
But there is a glass made by lightning strikes: Fulgurites.
They’re what you get when lightning strikes wet sand.
Fulgurites are usually hollow tubes of fused silica, coated in sand.
And they’re formed underground, sometimes running several meters in length.
That’s because wet sand conducts electricity.
And a channel of lightning only needs to raise the temperature of the surrounding silica
sand to 1800 degrees Celsius to melt it.
That might sound like a lot, but a typical strike can raise temperatures on the order
of 1,000 degrees Celsius per second.
So two seconds is all you need.
Fulgurites are usually tan or black, but it depends on the material they’re made from.
That includes impurities in the sand.
However, sometimes the lightning doesn’t strike sand.
It hits a silica-based rock.
Which still creates a fulgurite, just a non-tube-shaped one.
If there’s iron inside the rock’s crystals, the lighting can actually add electrons to
convert it into pure metallic iron.
So throw in some shiny silvery color, too.
Based on exactly what the lightning hits, there are four classes of fulgurite.
Type one are 95 to 100% glass, basically entirely sand and made of thin glass walls.
Type two are made from clay and type four are from rock.
Both are up to 90% glass, but have thicker walls.
In these, the main component of the glass is an amorphous silica-based compound called
lechatelierite.
Type three fulgurites, at the other extreme end, are only up to 10% glass, and mainly
composed of a natural calcium carbonate-based cement called caliche.
Fulgurites form underground, but as the sand shifts, they can start to peek out.
And when they do, we can identify trapped gas bubbles from the time they formed, allowing
scientists to learn things like what kind of plants used to exist in the area long ago.
The first glass sponges appeared over 570 million years ago, and their descendants are
still alive today.
Yes, living glass.
Sort of.
Glass sponges belong to the taxonomic class Hexactinellida, and are unique among sponges.
They have syncytial tissues, which means their cells aren’t specialized like ours are,
but rather merged together into one giant cell that can transport messages and materials
really quickly within the organism.
They’re found world-wide, usually at depths between 200 and 3000 meters.
Most are slow-growing, taking over two centuries to grow a single meter.
The largest living ones, off the coast of Canada and Alaska, can reach the height of
an eight-story building!
As the name suggests, their tissues contain amorphous silica-based structures called spicules.
In some species, they fuse together to form a glass-based skeleton that persists after
the sponge dies.
And in one species, the Venus flower basket, this glass cage actually traps a mating pair
of crustaceans.
They enter when tiny, but eventually grow to be too large to fit through the gaps!
But it’s not such a bad deal.
They clean the basket in exchange for food the sponges excrete as waste.
And one study of Venus flower basket spicules found that they transmit light similarly to
fiber optics but — because they form at low temperatures — have added ions that
make them better at it than traditional commercial fiber optics.
And they also have internal structural braces that make them less brittle.
So it turns out that Nature is far more inventive with glass than you might think.
But so are we!
Without glass you wouldn’t be watching this video.
Because humans took this simple concept of amorphous silica and crafted the vacuum tubes
that made computers run.
And later on used it to make circuit boards.
And monitors.
So let’s hear it for glass in all its forms.
Especially the living ones!
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