Welcome back everybody to Gemology for Schmucks. Today's episode is gonna be a
quick tutorial on how to use the refractometer. So last week in our
episode called "The Truth About Bling" I was talking about surface luster and
you saw the device the refractometer. So what I wanted to do is give a quick
tutorial on how to use the refractometer so that those of you who are aspiring
gemologists can have a head start and those of you that are not aspiring
gemologists can at least know how it works. If you're not interested in
technical stuff, now is a good time to click away and go watch one of my other
episodes. I hope that you have liked and subscribed already before you click away.
Alright, so here's how the refractometer works: let's get into it.
Here I want to tell you about how to use the refractometer for faceted gemstones
You can see here that we have a refractometer. The lens in the front can
be removed and this circular piece on top is the polarizing filter. So you can
see that there's a lid that lifts up that shields the reading of the refractometer
from outside light sources which can throw off the reading and you can see in
the middle there's kind of a rectangle of metal. Inside of that rectangle of
metal, there is a long rectangle of glass with a smaller whitish area in the
middle. That's very important. This is called the hemicylinder. Right here
we've got refractometer liquid. If you notice on here it's got the numbers 1.80
That is the highest refractive index reading that this liquid can take
There are higher and lower liquids but 1.80 is the most common for gemologists
It'll cover the wide variety of minerals that we use. Right here is a full
spectrum mag light that we use to generate light inside of the
refractometer. Other light sources can be used. I've seen my professors actually
use the lamp that's on their cell phone and it does work, but for the most
accurate readings you want to use full spectrum light. Right here in the middle
of the refractometer you can see the hemicylinder is now illuminated. That's
because light is coming from the back of the instrument and bouncing up through
this glass hemicylinder. It's of an absolutely essential part of how the
refractometer works. So in order to use the refractometer, we put a very very
small drop of refractometer liquid on the metal part of the refractometer here
Make sure that you screw the lid of the refractometer liquid on very tight
This isn't a very nice liquid. It smells awful, but it's very effective at doing
what it does. So we take our gemstone in question
here. We identify a part of it that has a very good polish. If the stone is not
polished you cannot get good enough optical contact with the refractometer
and you won't get a clear reading. Then we're gonna wipe off the gemstone that
we're going to test. Make sure there's no fingerprints or oils or any other kind
of substance on the surface of the stone: it will throw off the reading. Then we're
going to put that on top of a tiny drop a refractometer liquid that is on top of
the metal part of the refractometer and we're going to very gently, very
carefully scoot that on to the center part of the hemisphere once we've got
that in the center part of the hemisphere then we look down the
eyepiece. It's important to keep your posture your head your neck all in the
same location because you're not going to be able to read the refractometer
from all directions. At the top it starts with 1.30. That's a very very
low refractive index and at the bottom it goes all the way to 1.80
and you can see there are a couple of tick marks down below 1.80
What we need to do now is we need to look down the eyepiece and we need to
locate the gemstone and it'll be more of like an outline of the gemstone you need
to move your head up and down in order to see where the stone gets kind of cut
in half by that black line. That dividing point between the dark and the light is
where we're going to see our reading for the refractive index. Once we locate the
dividing line we keep our head in one location. If you move your head at all
once you've seen this stone, sometimes the gemstone will go out of sight and
it's going to be impossible to read the refractive index. Keeping our eye on the
area where the light is split into dark and light areas on the gemstone, then we
can pick up the lens and put it on the refractometer. The lens is going to
magnify the reading so it's easier for us to see and then we're going to put
the polarizing filter on top of the lens we're going to take our first reading of
the refractive index. So at the bottom of that dark line we're gonna read what
that number is and write it down. Then we're gonna take the polarizing filter
which is now on top of the lens and we're gonna rotate it at least 90
degrees. maybe 180 degrees... Frankly I don't pay attention to it. I just rotate
it maybe two three times fully around and you see whether or not that black
line jumps, and it should jump up and down every 90 degree turn of the lens IF
the stone is birefringent. So that means the light is being split inside of the
stone. That proves to us that this stone is birefringent. That's an important
piece of information for identifying what this stone is. If that
line does jump, you want to record what the other number is, because this will
help us get an idea of how large the birefringence is: what is the gap in
between these two readings. But once we've written down those two numbers, if
there are two numbers, if it doesn't jump at all you're only going to have the one
number and that's okay. We need to continue checking the stone and make
sure that we're not missing something by maybe we're looking at the c-axis so the
light even in a doubly refractive stone, or a birefringent stone, light won't
split in that direction, right? So how do we do that? We rotate the stone, so if the
stone is facing up and down like towards us and away from us, then you're going to
rotate it maybe 30 degrees to the right. It doesn't matter which direction but
you need to rotate the stone and do the exact same thing that we just did.
we look at the line. See where that is, mark that number. Rotate the polarizing filter
and see if it jumps. If it jumps, write down the other number, and you want to
continue this process. Rotate the stone, turn the polarizing filter and write
down both numbers until the entire gemstone has rotated 180 degrees. If the
stone is birefringent, then somewhere in this space we should have seen the
maximum birefringence. In different directions the birefringence will seem
larger or smaller. We want to find that direction that is the largest.
Whether or not the stone splits light is one important thing, and then the question
next is how much does it split light. That's its full birefringence and we
need to write down that number. So what is the range of refractive indices and
then how large is the birefringence. So the high refractive index... the low
refractive index, and then what is the space in between those two refractive
indices. We write that down and that is one of the most important pieces of
information for identifying gemstones. Once you have finished testing you wanna
be very careful about removing the gemstone from the hemicylinder, so scoot
it off carefully. Make sure you don't apply any downward pressure on the
gemstone, so it doesn't scratch the glass. Once you have removed it make sure that
you wipe off the refractometer liquid on the stone and on the hemicylinder.
Wipe forward to back because sometimes the refractometer liquid can crystallize a
little bit just in air and if there are crystals in between the
glass in the tissue and you press down too much, it can scratch the glass as
well. You have to be very careful about this
so what happens if a stone is a high refractive index, a high luster. We've
said that only up to one point eight can we actually read refractive index of
these stones. So if it's above the refractive index that the refractometer
can read, then when we look down the eyepiece, you'll see that this area
is entirely flooded with red or a rainbow color and as we move our head up
and down looking for that dark area that division between light and dark you
won't see it all you see is all red or all yellow or maybe a rainbow up through
the highest areas on the ruler of the refractometer. So this one we cannot read
We just record that it's OTL: over the limit, and that'll help us in
distinguishing what this gemstone is but not as clearly as having a refractive
index. This is part of why it's so important to look at your stone before
you try and take a refractive index reading because if it's a high luster
stone there's a strong chance that it's going to be over the limit. Remember that
high luster is connected with a high refractive index. That's why it's
important to use our eyes as well as using tools like a refractometer
For the spot test, this is typically for cabochons, so anything with a domed face. What we're
going to do first is rotate the stone around and inspect the surface and we're
looking for an area that has the best polish. Anything that's rough
or worn is not going to get a clear reading on the refractometer. It has to
be well polished. Once we've identified an area that has a good polish, then what
we're going to do is very similar to the faceted gemstone. We're gonna put our
torch in the back of the refractometer or whatever light source we're using.
We're going to open the refractometer hood, and put a small dot a refractometer
liquid on the metal part of the refractometer very very small amount.
For spot testing we're going to use as small as possible an amount of refractometer
liquid in order to get an accurate reading. This is more difficult than
testing faceted stones. So once we have identified the area of this stone that
we want to test, we're going to wipe off any sort of fingerprints or anything
like that it might be on this stone. Then we're gonna dip the part of the stone
that is highly polished in the refractometer liquid and we're going to
tap it several times on the metal part of refractometer. This is going to reduce
the amount of refractometer liquid on the gemstone. We need this to be a very small
dot because when you read a curved surface on the refractometer, if that dot
is not small, we're not going to get a good reading. So we're looking for a
small small small dot that is only the size of two or three of the smallest
dash marks on that ruler inside of a refractometer. Anything larger than two
or three dash marks and we're not going to get an accurate reading [of] what the RI
is of this curved stone. Same thing, we put the stone on top of that middle
rectangle on the glass hjemicylinder and then we move our head up and down until
we can see the dot. Then we're gonna write down that middle number. This is
what we call spot testing, and this is really the only way we can test the RI
of something that's cut in a cabochon. So in this situation with a refractometer we
cannot test birefringence of a cabochon stone. You'll need to use other
instruments. I understand this is highly technical, but if you watch this a few
times and you get your hands on an actual physical refractometer and a
whole bunch of different types of stones then you'll be able to practice and get
competent at this. It is a perishable skill set, so please keep in mind that
this is something that you need to practice often if you're going to be good at it.
People that are good at it and that don't do it for a couple of years
actually their skill set will reduce because remember it's how well you're
capable of locating that dark line division and once you've located that
dark line it's also important to be able to distinguish where on that dark line
the actual reading is but once you learn the general tips, of course that skill
set will come back quickly. There are different refractometers on the
market. There are some that are incredibly expensive, some that are
relatively inexpensive. If you're only going for faceted gemstones, then the
cheaper models are okay. There are certain ones that you can plug in and
they have the light built into them. Those are actually quite good: ONLY for
faceted gemstones, but if you want to be able to test the wide spectrum of
gemstones I absolutely suggest getting, if not the
gempro, which is what I was using, if you're trying to test all sorts of
gemstones definitely get one that the lens can be removed and that has a
polarizing filter. Those are both important features. You can use whichever
refractometer that you prefer. Big names and expensive models are not always the
best ones you know who I'm talking about. The one that I use is a few hundred
dollars less than perhaps the most famous of refractometers, but I prefer it
over the most famous. I'll put a link in the description for the gempro refractometer
and you can contact the maker directly. The maker of the gem pro refractometer
is actually in the United States, so if you're in the United States,
it's a great way to get an excellent piece of machinery and support
your nation [HUH!] and so that's all I've got for today. Thank you for joining me.
I hope that this has enriched your knowledge set and those of you that are
aspiring gemologist: Good luck. Get out there. Get some practice in, and you too
will climb the ladder of might and I'll see you next time ~