Welcome back everybody to Gemology for Schmucks. My name is Peter Nelson
and today we're gonna talk about the mysteries of light and gemstones. Since
the beginning of time people have been terrified of one special thing when
buying gemstones and that's to get a piece of glass when you think you're
getting a gemstone. It's humiliating and it's impoverishing. How can we
distinguish between glass and a lot of gemstones. Well there's a property of
light and the way it interacts with a lot of crystals that we're gonna learn
about today. When light goes into glass it stays the same. It picks up the color
of the glass then it exits the stone. That's fine
But with many gemstones, not all, what happens is that when light goes into the
stone, instead of just being one ray, it will split. Now not all crystals are the
same. There's seven or eight different classes of crystals, and what that means
is that on a tiny tiny tiny level almost atomic not quite there's something
called a unit cell. It's the basic block that the chemicals of the stone are put
into, and as those grow that defines its shape. It also defines its properties.
Some of those properties are things like how it breaks light and yes many
gemstones do break light and they break them in different ways. That's part of
how we test stones to know which minerals are what. Aside from just using
our eyes, we have a whole host of different tools that we can use to test
the gemstones. So you may think back to you your physics or your chemistry
classes as people are talking about light and the way it interacts with
different materials. Now light is like a jellyfish. Light moves as a wave
sure, but it's not like a snake. It doesn't just go in one direction. Light
from the sun actually pulsates, kind of like a ball getting bigger and smaller
as it moves through space and time. As the jellyfish of light hits a crystal
what happens? It will split into two rays and it will get polarized. Now what does
it mean to get polarized? The place that most of us have heard of polarized is
actually in sun-specs. Now how do these work? This glass is actually designed to
eliminate the horizontal plane of light, because if you're outside and
you're around water or if you're around cars or anything that has any
kind of reflection, as soon as light... the jellyfish of light comes down and hits a
flat surface, it gets polarized which means that it's forced to travel in one
plane whatever that plane is it doesn't matter polarized just means that it's
forced to travel in a plane. In this case most of the time it's the horizontal plane.
In nature that's what happens. Gravity. Blame it. So when light hits the surface
of the water where the metallic surface or reflective surface, it will bounce and
hit our eyes and that's what we call glare. So polarized glasses typically are
polarized to allow in the vertical rays of light, but eliminate the horizontal
rays of light. Polarized! So, sun-specs can also help us in testing these gemstones.
If light is being polarized inside of the gemstone and different colors are
being created, because of the different paths that light is taking and it picks
up different colors in the stone, what we can do is actually wear our polarized
sunglasses when you rotate it 90 degrees you'll notice that the stone changes
color and that's because the light that's coming at the stone from all
directions is being filtered into only the vertical direction by these
sunglasses. So one direction of light will come to you from within the stone
That's one of the rays that's split and you'll see one color. Rotate it again...
you'll see a different color. That's only with stones that have strong pleochroism
Some stones will split light, but they won't have distinct colors in
the two rays and that's based on the properties of the stone. Anyhow!
So not all doubly refractive stones are going to show pleochroism. They may
split light. They may polarize it, but that doesn't mean that the color
difference is going to be distinct enough for you to see it with your eyes
or with something as simple as this tool right here. So this is a dichroscope, and
let me show you how it works. So as the name implies dichroscope: di- means:
in Greek chro/chrome is color, and scope is some sort of a lens. So how do we use
a dichroscope. Very simply what you do is you take the dichroscope. You'll see
that there is a a small window. It's a little black box
in the front of it. You put the dichroscope on the table of the stone. It doesn't have
to be the table, but that's a very common starting point and then
you look down the stone to see the windows and you look and see if those
two windows have the exact same color or if the color differs AT ALL. And what
that does is when you put a gem on top of it you'll see some colors coming
through it. The question is, are those colors the same, or are they different?
and we're going to give you some examples in a moment. If it differs at
all, you know you're dealing with a doubly refractive stone. That's a stone
that splits light into two wavelengths. Now depending on which direction you're
looking at in the stone sometimes you will only see one color regardless of
the stone it is. So what you need to do is you need to look at the stone from
maybe the table maybe also look at it directly from the girdle. If you look at
it from the girdle that's a different area on the crystal and crystals have
different properties depending on where they're looked at. You could also look at
it from the culet. Doesn't really matter as long as you change the direction of
viewing. Now why is this important? It's important to change the direction
of viewing because inside of a crystal there's one area called the C axis where
the stone will still seem like it's singly refractive. Light will not be
split in that direction, so you have to change the direction that you're viewing
the stone in order to make sure that you're not getting that C axis where the
light does not get split. Some stones the light doesn't get split at all, but in
all stones there is one direction that will make it seem that the stone does
not split light. So the big thing that I want you understand about this concept
of pleochroism is that when light is split and it's polarized, as these two
rays travel in different directions and different planes in the stone they can
pick up different properties of these crystals these crystals in different
directions have different ways of treating light and that's really what
shows us that this is not glass in any way. Well in crystalline material, when
the light is split and polarized it's traveling through different parts of the
stone, so that means that the light path is longer or shorter. If light is in the
stone for a longer path, it's going to absorb more of the colors the light
that's not absorbed by the stone is what we see is color right? So it stands to
reason that if these two rays of light are traveling at different angles inside
of the stone then one of them is gonna be longer one of them is gonna be
shorter. So we're gonna see that as different color. Not all crystals are the
same shape. In current scientific understanding we know about seven or
eight different crystal classes. That means they have different properties
and different... what's called an axis of symmetry. If they're not perfectly
symmetrical... Like diamonds are cubic crystal system. It's the only one that has
the same properties from every direction. So diamonds do not split light. Garnets
do not split light you would have to use different tests to distinguish these
from glass. So what I really want you to take away from today's episode is that
glass, while it can be beautiful, is not the same as a crystal. Glass will never
make light obey it in the same way that crystals do. Some of these things we can
see with our eyes. Some of these things we need tools for. Once you know how to
use these tools you can quickly and effectively rule out glass as one of the
fakes for the gemstone you're hoping to buy. Once again this does not work for
all types of gemstone. There are types of natural glass. There are some gemstones
like garnets and diamonds that do not split light. Hopefully we can talk about
ways we can distinguish those from glass in a different episode. So just
remember that when the jelly-fish of light hits the crystal and it breaks
these two polarized rays that are traveling in different planes will pick
up different properties of the stone and that may give us the idea of pleochroism
So that means that it's picking up different colors inside of the
crystal. If it does this this is something that glass cannot do. So that's
all that I've got for today. Hopefully I haven't put you to sleep
with all the physics, but I love you. Take care of yourselves and I hope that this
episode has empowered you to go out and look at stones a bit more bravely thank
you for joining me on geology for Schmucks. Once again I'm Peter Nelson and
I appreciate you taking this time to learn about gemstones with me I'll see
you next time. Buh-Bye