People value gemstones for all sorts of reasons.
They’re usually rare, pretty durable, and
most of all, they’re shiny and sparkly.
They can have multiple colors, streaks of
light, weird inside-out shapes, and all kinds
of other qualities.
The things we consider gemstones are often
made up of minerals arranged into different
types of crystals, although a few are made
up of molecules that are arranged totally randomly.
But either way, their properties come from
their specific chemical structure.
At the atomic level, it’s simple geometry.
But it leads to some of the most beautiful
natural materials on Earth.
When you look at a gemstone, probably the
first thing you notice is its color.
Some gems, like tourmaline and fluorite, can
come in practically any color you can think
of.
And often, those colors come from transition
metals that are incorporated into the mineral’s
crystal structure — metals like copper,
iron, and zinc.
The transition metals are the metals in the
middle part of the periodic table.
Those elements tend to take on bright colors
because the way their electrons are arranged
lets them absorb visible light with certain
wavelengths.
With those wavelengths gone, we see a complementary
color.
Sometimes, a specific metal is an inherent
part of a mineral’s structure, so the mineral
always takes on that color.
Malachite, for example, has copper in its
chemical structure, which turns it green.
Other times, the metals aren’t an inherent
part of the mineral.
Instead, they’re sort of hitchhiking in
its crystal structure, occasionally taking
the place of whatever element would normally
be there.
Ruby and sapphire, for example, are actually
the same mineral, called corundum, with a
chemical formula of two aluminums and three oxygens.
When some iron and titanium atoms replace
a few of the aluminums, the mineral is brilliant
blue. That’s what we call that a sapphire.
But when the hitchhiking atoms are chromium
instead, the mineral turns red and you have
a ruby.
But there’s more to this color thing, because
gems aren’t always just one color.
Some look like they change color when you
view them from different angles, an effect
that’s known as pleochroism.
See, crystals are made up of atoms arranged
in repeating patterns.
The patterns form blocks called unit cells,
which can be different shapes, like cubes
or pyramids.
Geologists classify crystals by assigning
three axes to the unit cell.
Depending on whether those axes are at right
angles to each other or not, and whether they’re
the same length or not, crystals will have
different shapes and properties.
It’s kind of like the difference between
building a tower with cube-shaped blocks and
building one with pyramid-shaped blocks.
They’re gonna be different.
If the axes are all the same length and at
right angles to each other, like in a cubic
unit cell, nothing very interesting will happen
to the light passing through the crystal.
But when the axes are different, the light
can get split into multiple paths as it travels
through the crystal.
Different paths might absorb more of different
colors of light.
So for example, light traveling along one
path might seem more green, and along another
it might look more brown.
And when you rotate the crystal, you change
the paths the light takes.
So pleochroic crystals seem to change color
as you move your head, or rotate them in your
hand.
Depending on their exact geometry, you can
get either two or three different colors.
Not all non-cubic gems are pleochroic, and
even when they are, the color change isn’t
always noticeable to our eyes.
But it’s a pretty common property.
Sapphire, for instance, is often pleochroic.
So is topaz.
It’s just light interacting with atomic
geometry.
But it looks awesome.
Then there are gems that are attracted or
repelled by a magnetic field.
You can’t just walk up to your refrigerator
and stick a garnet to it or anything, but
strong magnets will attract certain kinds
of gems.
In fact, the same transition metals that give
gems their color can make them drawn to magnets.
Often, the metal responsible is iron, but
rare earth elements like neodymium can do
it too.
Those trace elements make the mineral magnetic
because they have odd numbers of electrons.
Electrons have a property called spin, and
two electrons with opposite spins pair up
and cancel out.
But when an electron isn’t paired, its spin
goes un-canceled, and it’s free to be attracted
by a passing magnetic field.
That’s called paramagnetism.
On the other hand, a material with all its
electrons paired up is slightly repelled by
a magnetic field, because when the electrons
move around in an atom, they make magnetic
fields of their own, which repel other magnets.
That’s diamagnetism.
Bismuth, for instance, is diamagnetic, so
it’s always repelled by a magnet.
When unpaired electron spins line up parallel
to each other, you get ferromagnetism, or
a mineral that’s an actual magnet.
Very few minerals are ferromagnetic, but hematite
is one example.
Sometimes, a gem that’s normally repelled
by magnets will have bits of iron inside it,
which will make it attracted to magnets instead.
So magnetism isn’t a perfect tool for figuring
out what a gem is made of.
But it’s often a helpful clue.
You’ll often see gems cut into facets, but
other times, they’ll be polished into a
round shape called a cabochon.
And sometimes, a round-polished gem will look
like it has a bright streak of light across
its surface, like the vertical pupil in a
cat’s eye.
It’s called chatoyance, and it happens because
of little thread-like pieces of a mineral,
like rutile, inside the gem — what scientists
call silk.
As these crystals form, the impurities are
forced to line up along the axes of the crystal’s
structure, so the pieces of mineral end up
parallel to each other.
Those parallel pieces reflect light in a way
that creates a bright line perpendicular to
the threads.
And it’s not just gems that do this — a
spool of silk thread will do the same thing,
where there’s a streak of light perpendicular
to the wound-up thread.
But unlike a spool of thread, gems can have
inclusions going in different directions,
based on the crystal structure of the mineral.
That creates a streak of light perpendicular
to each axis, which looks like a star with
four, or six, or even more points.
The star effect is called an asterism.
When a gem has these threads, cutting it into
facets might make it look kind of muddy.
But when it’s polished into a round shape,
you get gorgeous streaks of light.
Polymorphic minerals don’t always have the
same structure.
Even if they have the same chemical composition,
the temperature and pressure when they form
can lead to different shapes.
Carbon is probably the most famous example
of this.
Depending on how its atoms are arranged, carbon
can form soft, slippery graphite or basically-indestructible diamond.
Silica, the mineral that makes up sand and
quartz, also has lots of different polymorphs.
Its molecular formula is always the same:
one silicon atom and two oxygen atoms.
Its molecules form tetrahedral shapes -- that’s
a triangular pyramid, or a d4 if you’re
into tabletop RPGs.
Tetrahedrons can stack into different shapes
as changes in temperature and pressure juggle
them around.
At the temperatures and pressures humans find
comfortable, silica makes the alpha, or so-called
“low”, form of quartz.
[kriss-TOH-buh-lite], [STISH-uh-vite]
But as temperature and pressure increase,
it can become things like cristobalite, which
is found in lava flows, or stishovite, which
is in meteorite craters.
Even a single polymorph of silica can take
on a huge variety of forms.
Alpha quartz can look like scepters, rounded
pebbles, or clusters of needles.
It all depends on the growth conditions: like
how fast the crystals form, how much space
is available, and how much material there
is to make crystals.
The unit cells stack together in the same
way, but sometimes an impurity will cause
them to take off at an angle or make them
more likely to stick to one part of the crystal
than another.
Quartz is so varied that tons of gems -- like
amethyst, chalcedony, agate, and citrine -- are
all made of silica.
Organic matter can be slowly replaced by minerals
to become a fossil.
Often, the mineral involved is kinda drab,
but sometimes the conditions are just right
to produce something spectacular.
And in rare cases, petrified wood, shells,
teeth, and even bones of extinct organisms
are made of opal.
Opalized fossils are most often found in Australia,
along with most of the world’s opal in general.
One of the most famous specimens is a pliosaur
nicknamed Eric, an almost-complete marine
reptile preserved in opal.
Like quartz, opal is made of silica.
But unlike quartz, it doesn’t have a crystal
structure.
Instead, it’s made of little spheres of
silica all bunched together.
These tiny spheres scatter light, giving opal
its characteristic rainbow sheen.
Geologists have a few different models for
how opal might form, but it could come from
silica weathering out of rocks in an acidic
environment.
Australia used to be partially covered by
an inland sea.
And as that sea dried up, it left acidic,
silica-rich gel behind.
Bits of silica settled into the gel and then
grew into the spheres that make up opal.
Sometimes, that gel was stuck in bones or
bits of wood that had already started to fossilize,
so the silica trapped in there formed opals
in the shape of those organic structures.
The resulting fossils have both aesthetic
and scientific value, and in 1993, Eric the
pliosaur was almost made into jewelry by a
broke owner looking to sell and potentially
break up the pieces.
But a crowdfunding campaign rescued Eric and
got him a place in the Australian Museum.
Hopper crystals are probably some of the strangest-looking
crystals.
They’re shaped into a weird stair-stepped,
hollow kind of pyramid known as a hopper.
The shape comes from a quirk of chemistry
as the crystal is forming.
When the growth rate and saturation of the
crystal-forming solution is just right, new
molecules will tend to be more attracted to
the edges of the growing crystal than the
inner flat surfaces.
That makes the edges grow out of control while
the flat faces mostly stay the same, so the
crystals grow in a lopsided way that leads
to that fascinating inside-out shape.
Bismuth hopper crystals have to be grown in
the lab, but hopper crystals have also been
found in nature, in minerals like rose quartz.
Bismuth is pretty easy to get your hands on
and has a low melting point, so you can actually
try to make your own hopper crystals, if you’re
feeling adventurous.
Some materials that are culturally prized
as gems aren’t minerals or gems in the strictest
geological sense: things like amber, jet,
coral, and pearl.
Instead of being made up of crystals, all
these materials have more amorphous chemical
structures, and they come from living things
— that’s why they’re called amorphous organics.
But the way they form makes them look a lot
like the classic crystal gems.
Amber is fossilized tree resin, and can actually
preserve organisms that get stuck in it.
When the resin gets buried in calm, wet, low-oxygen
environments, it slowly turns to amber.
Jet is practically the same thing as coal
in some ways.
But while coal forms in big seams from huge
amounts of plant material, jet is formed from
small bits of wood that get buried in sediment
and compacted.
Jet can be cut and polished to a gem-like
shine.
But it doesn’t have a rigid crystal structure
— at a microscopic level, it can actually
preserve the cellular shapes of the plant
it used to be.
Coral is made up of small colonial animals,
with calcium carbonate skeletons.
The skeletons are usually white, but the precious
coral that’s often considered a gem is a
species that includes reddish-orange carotenoid
pigments.
Pearls contain a mixture of protein and calcium
carbonate secreted by certain types of molluscs.
Different species will produce different colors
of pearls, and impurities in the water can
also affect the color.
Like all of the gems on this list, amorphous
organics look the way they do because of the
way their atoms are arranged.
The regimented structure of a crystal and
the laid-back chaos of an amorphous solid
both affect the way they interact with light,
magnets, and other materials.
It’s the sparkliest kind of geometry.
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