tools you'll need:
A polariscope is a simple tool that will help you determine if your gem is doubly refractive (DR), singly refractive (SR), or an aggregate (AGG). Note that only transparent and translucent gemstones should be used with a polariscope; opaque or nearly opaque stones won't provide a reading.
To use a polariscope, turn the light on and place your stone face down on the polarizer (the lower glass lens). Look down onto the stone through the analyzer (the top lens), turning the analyzer until the area around the stone looks the darkest. This is your starting point. Watching the stone, turn the analyzer a full 360 degrees, until the area around it becomes light and then returns to its darkest again, as shown.
Tip: If you have trouble seeing the light/dark change in your stone, pierce a small hole in a white card and place it over the analyzer, as shown, to block out some of the distractions and light. Then peer at your stone through the hole and attempt to take another reading. A similar trick might also help you focus your eyes on the most transparent area of the stone if it is heavily included.
If your stone appeared dark and seems to remain dark, it is probably SR, though it might be DR. In order to be sure, you should turn the stone a bit and take another reading, twice, checking the stone from at least three views. If it's still dark from all three views, it is SR. Once you've determined that for sure, stop with this test and move on.
If your stone started out light and appears to stay light, it's probably AGG. Various factors can cause a DR stone to appear AGG, like inclusions or high RI readings.Your stone might "blink" from light to dark back to light, or vice versa. In that case, it could be either DR or SR with ADR, which means anomalous (false) DR. In this case, further testing is required. ADR stones include amber, opal, diamond, synthetic spinel, and garnets with red in their colors (orange, purple, red), as well as glass. While the stone appears "light," you might see a "crosshatch" appearance. This is an indicator of an ADR and a good clue that the stone might be synthetic spinel. If you see dark and light bands that appear to move and even cross as you turn the analyzer lens, you probably have an SR stone with ADR.
Stones with a high RI reading should be tested on their sides rather than face down to cut down on light bouncing around of off their facets and to provide the best results.
Small stones might be hard to read properly. Try using the magnifying lens from your refractometer on top of the analyzer lens to get a better look. Also try to look at only the most transparent areas of your stone if it's heavily included.
If your stone is red and you found a refractive index of 1.73 or greater, skip the polariscope and just use the dichroscope for this step.
Make Your Identification!
Congratulations! In most cases, you now have gathered enough information to determine your stone's identity. Find the gemstones that have your mystery stone's color and RI. In many cases, that alone will positively identify your gem. According to Boyle, "You can clearly identify 95% of all gems with one simple test. Each gemstone has a unique RI that can be proof positive to identify the gems you have in your parcel."
Match your information to the data on our Gemstones 101 pages or to the RI, birefringence, and other charts in Walter Schumann's book, Gemstones of the World.
However, if that leaves you with more than one possibility, you can narrow your options with birefringence and double vs. single refractivity. Only a small percentage of gemstones will still be a mystery after these tests, and more advanced analysis and details will be required to positively identify them. You can learn the more advanced steps through GIA's Gemstone Identification course (available even through the mail or online) and week-long on-location lab class, like I did, or consult books like Antoinette Matlins' Gem Identification Made Easy and Richard T. Liddicoat's Handbook of Gem Identification. Boyle recommends both, sharing that "both books are excellent in walking the novice through the steps to identify a gemstone."