Aquamarine is the birthstone of March. Its colour ranges from light blue to blue-green to a deeper blue. The deeper blue is often achieved by heating the stone to 398 degrees C. Blue topaz is the birthstone of December. Although blue topaz and aquamarine can look identical, they come from different crystal families, and aquamarine is often considered more desirable and is therefore more expensive. Using an aqua filter makes it possible for you to determine whether your stone is an aquamarine or a blue topaz.
Determine whether the stone was labelled aquamarine or blue topaz when purchased. Blue topaz is often sold as the more expensive aquamarine.
Inspect the stone for visible flaws. Flaws can be scratches, rough marks or discolouration that appear on the surface of the stone or within it. If the stone is flawed, it is likely a blue topaz. Aquamarine rarely has visible flaws.
Observe the colour of the stone by holding the aqua filter very near your eye and holding the stone close to the filter under a bright incandescent light. Looking though the filter --- depending on the size of the stone --- you may be able to see some green and blue tones, indicating an aquamarine, while the blue topaz displays only blue tones.
Place the stone on the microscope. Observe the stone using 20 times magnification. You will clearly see that the aquamarine displays green tones along with its more prominent blue tones. Blue topaz will only show blue tones.
You can purchase a gem filter set for about 65 dollars.
Using only your eyes and an aqua filter --- without a microscope --- does not yield guaranteed results.