All photographs by Robert James FGA, GG. ©2014 YourGemologist.com
What is it?: A polariscope is a gemological instrument that tests for a gemstone being double or single refractive, and will allow us to find the various crystal axis of the stone. Which is where is got its name: Polar-scope..the a scope to see the poles or axis of the stone. Now, if you remember the section on the dichroscope, the use of plane polarized light allows us to see the different colors of light being transmitted by a gemstone. The polariscope, however, allows us to actually see the path that those beams are taking through the stone. By knowing that the stone is double refractive, we can use the optic interference figure to actually find the various optical directions that the light is traveling through the stone, and thereby make identifications based on this information.
Let's clarify this concept. As we found with the dichroscope, single refractive gemstones do not break the light beam up into various colors. Meaning that one beam in and one beam out. There are relatively few gemstones that are single refractive, and identification of those stones is fairly easy owing to their rarity. There are a lot of double refractive gemstones though. Some break up a beam of light into two beams...like ruby, sapphire and emerald...while others break up a beam of light into three separate beams...like tanzanite and andalusite. What the polariscope does is allow us to see the different directions that the light is traveling through the stone. How? By allowing us to see the optic interference figure that the light beam creates as it travels through the stone.
It is truly a defining moment in the life of any gemologist when they first are able to find and identify their first optic interference figure. The first time you actually see a quartz bulls-eye you will (if you are like me) sit back in your chair for a minute and contemplate the wonder of not only seeing this figure, but realizing that what you are seeing is the path of a beam of light through a gemstone. This is a truly wonderful event...even if some of the old timer's are too far removed from their own event to remember. I remember...and it was 28 years ago that I found my first uniaxial interference figure. Admittedly, it did not rival seeing my son take his first breath....but it was an astounding event nonetheless.
There is a lot of technical gemology that we do not have room to cover here regarding the polariscope and what it is actually telling you. But if you will take a few minutes and learn what it is saying, perhaps later we can spend more time and understand the why it is saying it. But for now, learn the what of the polariscope, it will allow you to make gemstone identifications from a very early time in your study. And we will take up the why in the course notes of the International School of Gemology. But for now, let's learn the parts of the polariscope:
Why it works: As stated above, the polariscope works by transmitting plane polarized light into a gemstone. When you turn both of the polaroid plates to the dark position, a single refractive gemstone will not change the path of that light, and the stone will remain dark in all positions. With double refractive stones, however, the stone will actually change the path of the light, thereby changing the direction of the wave of light, and create a plane of light that is no longer polarized. It is this beam of light that has had its direction of travel changed that makes the stone turn light to dark as you rotate it between the polaroid plates. This is how the polariscope works. But wait...it does more. With the use of a conoscope you can actually see that beam of light travel through the stone in what we referred to above as an optic interference figure. The conoscope is a simple glass sphere that has no internal strain to give false strain readings as shown with the amber below. With the conoscope, and some people can do the same thing with a 10x loupe, you can actually see the optic interference figure, which will tell you whether a stone is uniaxial or biaxial.
You can purchase a professional quality polariscope at a special visitor price in the YourGemologist Gemology Equipment Store by clicking on this link:YourGemologist's Polariscope.
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Robert James FGA, GG.....YourGemologist