What Is A Reflectometer?


“Faceted Diamond 1” by James St. John is licensed under CC By 2.0
The table is the large, horizontal facet on top of a gem that acts as a window to its interior. Viewing an expertly cut and well-polished facet is essential for getting an accurate reading of a gem’s luster with a reflectometer. “Faceted Diamond 1” by James St. John is licensed under CC By 2.0

Question

In my quest for a refractometer, someone suggested that I purchase a reflectometer as a more affordable alternative. However, I’m having difficulty getting much information on this tool as it relates to gemstones. Do you know anything about reflectometers? Is this one of the devices you discuss in your review of inexpensive refractometers?

Thanks for your help.  Your knowledge base is enviable!

Patty

Answer

A refractometer measures refractive index. A reflectometer measures luster, how a gem surface appears, and is usually calibrated to tell you what a stone “most likely” is. For example, if you’re looking at a ruby, a reflectometer will quickly tell you if the gem is not corundum. However, that is far from what you need for positive gem identification.

Reflectometers are also notoriously inaccurate. Their accuracy is based on the quality of the facet you are reading.  I have had two and they both gave me birefringence readings on the table of a one-carat diamond. You won’t find a flatter or better polished table than that. And diamonds have no birefringence!

Donald Clark, CSM IMG