Thanks to Brilliant for supporting this episode
of SciShow. Go to Brilliant.org/SciShow to learn more. [ ♪INTRO ] 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! If you haven’t had enough bite-sized learning
for today, you might be interested in an extra challenge: Brilliant.org offers Daily Challenges
that are posted every day, with the goal of teaching you something about everything from
statistics to electricity to computer science. Each challenge question comes with illustrations,
animations, or interactive visualizations, and all the context you need to solve the
problem yourself. Better yet, each one ties into one of the
50 courses offered by Brilliant, so if you like one, there’s tons more to learn. You can even tackle the entire archive if
you have a Premium subscription. And right now, the first 200 people to sign
up at Brilliant.org/SciShow will get 20% off a year’s worth of the Premium subscription. Which is a pretty neat deal — so see if
it’s right for you. And thanks for your support! [ ♪OUTRO ]
I enjoyed watching this, cool post.