Welcome back to Gemology for Schmucks everybody. Once again I'm Peter Nelson
and I'm here to talk to you about some of the secrets of gemstones. So last week
we talked about how when light goes from the air into the gemstone some gemstones
will split light and today we're gonna continue on that concept but with a
different effect that it gives us. So when light goes from one substance like
the air into another substance like the water it will actually bend, right? If
you've ever tried to grab a fish you know it can be incredibly difficult
because where we see the fish is not where it physically is located. Why is
that? Because when light goes from the air into the water, instead of going
straight, it actually bends. It slows down a little bit, because it's a more
optically dense material. In order to grab that fish you have to reach in, not
to this location where we see the fish but, over here. Same thing happens in
gemstones except there's no fish. So another thing that can happen yes
light is being bent but in doubly refractive stone, so that stones with two
rays of light, the distance between those two rays is called birefringence. Bi
meaning two and refringence, refringence and refraction are very similar so
refraction is how much light is bent so if it is doubly refractive or
birefringent, more or less the same thing, then if it splits light widely
enough we might get a double image on the other side of the stone. When you're
looking for doubling you're looking at one spot on the facets on the other side
of the stone and if you see the same image mirrored so there's two of them
then that is what we call doubling. If you see doubling what do we know? well we
know for sure that this stone is doubly refractive so if somebody is coming for
example and saying that oh this is a diamond and it has doubling, then it
can't possibly be a diamond because diamonds do not split light. If it's not a
diamond it can be a number of other stones that are white that do split
light. So it could be white zircon for example. It could also be synthetic
moissanite. Moissanite is typically not a natural stone. Now our human eyes have
limits and if the birefringence is too small then you're probably not going to
see the doubling effect unless you're a cyborg.
Some stones have a larger birefringence. Something like zircon for
example now all of these colors are zircon we've got the blue zircon here
we've got some other reds greens yellows there's also white zircon there's plenty
of different colors of zircon and one of the interesting things about zircon is
that the blue zircon has a very strong birefringence which means it also has a
strong doubling effect. You will also see a doubling effect in other stones with a
large birefringence something like this Turkish Diaspore. So right here as we
look down into the stone straight through the table we can see doubling on
the other side. Singly refractive stones like diamonds garnets or something like
glass will not do this. Now when you're looking for doubling you have to be
careful of one thing. As we said last week with pleochroism there are certain
angles in the stone that have what's called an optic axis sometimes we call
it the C axis and that is the angle the part of the stone that does not split
light and that also happens in doubly refractive stones. Stones that do split
light. If you're looking for this doubling effect you need to look at the
stone from different angles. Just like with pleochroism, certain angles if
it has doubling you will see it, certain angles even if it has doubling you won't
see it. So how do we check for doubling you start out with your tweezers right
here and you will also need a loupe. A 10x triplet loop that we talked about in
another episode. Check it out right here. The first thing that you're going to
want to do is use a nice cloth okay microfiber or the cloths that you use
to polish your glasses or a personal gemological favorite: ShamWow. (this is not a sponsored video)
you're gonna hold the stone typically in your left hand your non-dominant hand
and your dominant hand is going to use the loop just like we showed you before
Now when you're looking for doubling in the gemstone you can look straight
through into the table onto the other side and look at the back facets
depending on how the stone was cut, if the stone was cut and the c-axis is
right here, then these facets down here at the culet, we're not going to see
doubling. The next direction that you want to look is from these crown facets
through to the other side of the pavilion so that gives us more of an
angle through the stone if you don't see from the table side you may see it from
this direction. Now once again that's only if the stone has strong enough
doubling for you to see. You have to have an idea of what stone you're looking for
and whether or not its birefringence is large enough to show doubling. Okay so a
quick review: refraction is what happens when light goes from the air into a more
optically dense material whether it's water whether it's crystals whether it's
whatever when it hits that optically dense material it bends okay. Doubly
refractive stones split light and the angle between that split is called
birefringence the larger that birefringence the higher chance we have
something called doubling and if there is doubling we know that light is being
split and it's not a singly refractive stone. So a singly refractive stone only
has one ray of light that's being bent. So you have to know about the nature of
the stone that you're interested in buying. If you're looking for diamonds if
you're looking for white sapphires if you're looking for white zircon they're
all going to have different birefringences. Some of them you will
see a doubling effect. Diamonds you will not see a doubling effect. So this
is just one more piece of our toolkit that helps us to understand: is this
stone when people say it is or is it something else and it's all down to the
natural properties of that stone. So that's all I've got for today. Thank you
for joining me on Gemology for Schmucks. Go out there. Check out some more stones
broaden your mind and appreciate the treasure of this earth and I'll see you
next time, bye bye.