DCA Gemology Practice Test

Light movement inside a gemstone is governed by its refractive index, a measure of how much light slows and bends as it passes through. Diamonds have a notably high refractive index, around 2.417 to 2.419. That high value makes diamond stand out when you test a gem against common simulants, many of which have significantly lower refractive indices (like glass or cubic zirconia). By measuring the refractive index with a refractometer, a value near 2.42 strongly supports that the gemstone is diamond. This test is a practical first-step discriminator in identification, though it isn’t perfect on its own—some stones, such as moissanite, can complicate the picture and may require additional tests (like checking for birefringence) to confirm.