Gemologists must be able to tell the difference between real and fake gems that appear identical to the naked eye. One way to determine which gem is real and which is fake is to compare the stones' refractive indexes, a property of that is unique to each gemstone.
According to GeoGem, "the refractive index of a gemstone provides the single most important piece of information to a gemologist seeking to identify an unknown stone. It is a constant that is measurable to four significant figures, and can allow gems to be distinguished even when their refractive indexes differ only very slightly."
Remember that refraction is "the bending of light when it passes from a rarer medium (Air) into a denser medium (gem.)" Basically, the refractive index is a ratio that compares the speed of light in air versus the speed of light in the gemstone.
The equation for the refractive index is:
a) Based on the above equation, what is the relationship between the speed of light in the gemstone and the refractive index?
b) Given that the speed of light in the air is 186,000 miles/second, and the RI index of a diamond is 2.42, how fast does light travel in a diamond?
c) One method of determining a gem's refractive index is to immerse the gem into liquids with known refractive indexes, and to watch the effect of the liquid on the appearance of the gem. For this question, remember that refraction means that light bends as it enters different materials. What would the gem look like in a liquid with the same refractive index as the gem's?
To see the solution, click on the "Solution" link in the frame to the left.
As we all know, one of the most interesting aspects of a gem is its ability to bend, reflect, refract, and filter light. Of course, not all gems change light in the same way. Some gems have a property know as single refraction where the light is only bent as it passes through the rock. However, other gems have a property know as double refraction which means the light is
polarized into two rays at traveling at different velocities.
Using the polarized paper, look through the glass set between the two pieces. The two pieces are set at 90 degrees which means no light should pass through. Now rotate the GLASS and explain what you see.
Now follow the same procedure using calcite. Explain what happens as you rotate the rock. What happens and why? Compare what happened with the calcite to what happened with the glass. Hint: Remember the polarimeter experiment (the one with the syrup).
To see the solution, click on the "Solution" link in the frame to the left.
