00:00:00
In January 1848, a carpenter in
California found some flakes of gold
00:00:05
in the waterway supplying the
sawmill where he was working.
00:00:08
It was a lucky find, and before long,
00:00:10
thousands of other hopeful prospectors and thieves
00:00:13
descended on the area
00:00:14
to find their own golden treasure.
00:00:16
Thus began the Californian Gold Rush,
00:00:18
which saw more than 300 tonnes
of gold extracted from the area
00:00:22
around the Sacramento River.
00:00:24
While seeking their fortunes,
00:00:25
prospecting hopefuls needed to
be careful they weren’t tricked.
00:00:29
Fool’s gold, a shiny yellow
metallic mineral called pyrite,
00:00:33
was everywhere.
00:00:34
And it wasn’t worth a dime,
00:00:36
especially compared to the
gold in them thar hills..
00:00:39
But as it turns out,
00:00:40
there was another kind of fool’s
gold out there called chalcopyrite,
00:00:44
meaning there were twice as
many tricks to watch out for.
00:00:47
These were the days before fancy chemical tests,
00:00:50
they needed a rough and ready field test
00:00:52
to tell them they’d gotten the good stuff.
00:00:54
And it turns out that this is the
simplest and most effective test.
00:00:58
[intro music]
00:01:02
The California gold rush ultimately brought about a quarter of a million hopefuls
00:01:06
to the area,
00:01:06
hoping to strike it rich.
00:01:08
But gold fever wasn’t limited to America.
00:01:10
Around the same time, Australia
experienced its own gold rush,
00:01:14
and all through the second
half of the 19th Century,
00:01:17
gold hunters flocked to
locations in Canada, South Africa
00:01:20
and elsewhere in America at
the first whiff of a gold find.
00:01:24
Prospectors had to be reactive,
00:01:26
because it was hard to predict
where the gold would be.
00:01:29
Gold can be found in lots of different settings,
00:01:31
can exist as tiny flakes or giant nuggets,
00:01:34
and might show up in streams and lakes
00:01:36
or still be embedded in the rock itself.
00:01:39
It’s a moving target.
00:01:40
Part of the problem is how gold
gets to the places we find it.
00:01:43
See, gold is first deposited in hydrothermal veins
00:01:47
that cut through existing rocks.
00:01:49
But it tends to be in small
quantities and really spread out,
00:01:53
so it’s easy to miss.
00:01:54
But when those veins get
weathered and the gold washes out,
00:01:58
it gets redeposited in river and lake
sediments called placer deposits.
00:02:03
And here, among modern and ancient sediments,
00:02:06
is where you can find amounts
of gold worth your time.
00:02:09
The thing is, the geological processes
00:02:12
that help to concentrate
gold into valuable deposits
00:02:15
also tend to gather together other minerals,
00:02:18
like pyrite and its cousin chalcopyrite.
00:02:22
Chalcopyrite is the fool’s
gold you’ve never heard of.
00:02:25
It’s similar to pyrite, which has
isometric crystals made of iron and sulfur,
00:02:30
but it has extra copper in it,
00:02:32
which makes these tetragonal crystals
00:02:34
that like to grow together into one another.
00:02:37
When chalcopyrite reacts with oxygen in the air,
00:02:40
it can also turn a bunch of psychedelic
colors, from green to peacock purple.
00:02:45
But in its natural un-oxidized
state, it’s a brassy gold.
00:02:49
Not only is it gold in color,
00:02:51
but it’s pretty soft as far as minerals go.
00:02:54
You can scratch it with a copper coin,
00:02:56
which means it can look about as soft
as real gold to the untrained eye.
00:03:01
Both pyrite and chalcopyrite occur alongside gold
00:03:04
in old hydrothermal systems,
00:03:06
where metal rich fluids have
flowed through existing rock
00:03:10
and left behind their mineral treasure.
00:03:12
It’s little wonder that these two fool’s golds
00:03:15
have wound up confusing optimistic prospectors.
00:03:18
Like Jaques Cartier.
00:03:19
Cartier - no relation to the French jewelry guy -
00:03:23
was exploring the St. Lawrence
River in Canada, in the 1540s,
00:03:27
when he found what he thought
to be gold and diamonds.
00:03:31
It was only when he got his
treasures home to France
00:03:33
that he discovered that it was
just a bunch of quartz and pyrite.
00:03:37
So it goes to show that real people
00:03:39
have been actually fooled
by these lookalike minerals.
00:03:42
Huge Bummer dude
00:03:43
And even during the later gold
rushes of the 19th Century,
00:03:47
prospectors would have needed
quick ways of distinguishing trash
00:03:50
from treasure in the field.
00:03:52
So, in lieu of any fancy geochemical analysis,
00:03:55
rock hunters could have used a streak test,
00:03:58
to tell all of their shiny golden minerals apart.
00:04:01
All you need to do is treat
your rock like a crayon.
00:04:04
Just drag it along an unglazed
white ceramic surface,
00:04:08
and look at the color of the streak left behind.
00:04:11
And you might be surprised,
because a mineral’s streak
00:04:13
isn’t always the same colour
as the rest of the crystal.
00:04:17
That’s because the color of a crystal
depends as much on geometric structure
00:04:21
as it does the chemistry.
00:04:23
Minerals like opal and labradorite
owe much of their color
00:04:26
to the way light bends through their structure.
00:04:29
And trace elements contaminants
can also throw off the color too.
00:04:33
For instance, calcite is a mineral
00:04:34
that comes in lots of colors
depending on what trace impurities
00:04:38
are caught up in the crystal,
00:04:40
but no matter the color of the whole piece,
00:04:42
its streak will always be white.
00:04:44
So doodling with your rock removes the
diffraction and contamination factors,
00:04:50
meaning that the streak color
will be the same no matter what.
00:04:53
Of course, the streak test only works
00:04:55
if your mineral is softer than the
ceramic tile you're dragging it across,
00:04:59
so about a 7 on the Mohs hardness scale.
00:05:02
If they’re not, then all you’d
get is a scratch in your tile.
00:05:05
Which, conveniently enough,
applies to all kinds of gold -
00:05:08
foolish or otherwise.
00:05:09
are all lower than seven on the moh's hardness scale
00:05:12
Even though gold and its lookalikes all
have softish, shiny yellow crystals,
00:05:17
they have different colored streaks.
00:05:19
Gold, fittingly, leaves a golden
yellow streak on the plate.
00:05:23
On the other hand, pyrite’s streak
00:05:25
is a greenish, somewhat metallic black.
00:05:27
And as for chalcopyrite?
00:05:29
The streak is greenish black, just like pyrite.
00:05:33
All it takes is a ceramic tile and a pair of eyes!
00:05:37
And while chalcopyrite might not have been
00:05:39
what the gold rush prospectors wanted to find,
00:05:41
it’s not exactly trash, either.
00:05:44
Today, it’s one of the main
ores for all the copper
00:05:46
we use in our pipes, wires, and construction.
00:05:49
It’s also used to make sulfuric acid,
00:05:52
which has a role in fertilisers
and industrial chemistry.
00:05:55
And places like Copperfields Mine in Ontario
00:05:58
and Olympic Dam in Australia
00:06:00
are now better known for their
copper-bearing chalcopyrite
00:06:04
than for the gold that’s found alongside them!
00:06:07
Who’s the fool now?
00:06:08
What’s even better,
00:06:09
Rocks Box subscribers get a piece
of this beautiful golden mineral
00:06:14
with its green-black streak, right to their door.
00:06:17
Every month we pick out a
special mineral or fossil
00:06:20
with a story to tell and send them your way.
00:06:22
If you wanna get on the
waitlist or peruse our shop
00:06:25
from some of our greatest hits of the past,
00:06:27
head over to SciShow.rocks or
click the link in the description.
00:06:32
Thanks for watching!
00:06:33
[ OUTRO ]
