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Refraction
When light travels through an object, it bends, or is refracted at an angle. It changes both directions and speed, and you can measure the refraction to determine the angle. Reflection is another measurement of light, which is the light that bounces off the surface of a gem. Each gemstone has its own refractive index, which is a number that represents the angle that the light is refracted at for that gemstone. You can use the refractive index to identify different kinds of gems. Refractors measure a refractive index up to 1.81 usually, which can exclude some higher stones like diamonds.
Birefringence
Materials with a crystal lattice formation can exhibit birefringence. This means that as the light passes through the gem, it splits, and one part is refracted while the other passes straight through. An example of a gem that would show this would be quartz. Almost all refractometers can measure this, which will give a more accurate refractive index for the gem.
Refractive Index
The refractive index is a calculation involving both the incident angle and the refractive angle. The incident angle is the light's angle as it enters the gem, and the refractive angle is the angle that light bends at once it is inside the gem. The ratio between the two angles will equal the refractive index, which is different for each mineral.
Gem Identification
A refractometer measures the refractive index of the gem. You use refractive index liquid to eliminate air between the refractometer and the gem so the light will pass straight into the stone. The refractometer shines the light through the stone, and you can see it on a scale through the eyepiece. There are many other tests that determine the type of gem without a refractometer. You can test hardness, color, and morphology of the stone, all of which will help you get an even clearer view of what you have.
Alternative Meters
In addition to refractometers utlilized for gem identification, there are also gem filters and reflectivity meters. Gem filters are the least expensive. When looking at a gemstone through a gem filter, you will only see certain colors of the light, which will help to identify the stone. Reflectivity meters measure the reflectivity of the stone rather than the refraction index. This is less exact than refractometers, but is still very useful in identifying gems. Lastly, there are gem testers that measure thermal conductivity of stones. You can use them on more types of stones than refractometers or reflectivity meters.