Step 3: Practical Gemology

Lesson 1

What is a Dichroscope?

Dichroscopes are relatively inexpensive instruments. You can even make your own. There are two types: one has two pieces of polarizing filters at one end (sometimes called a “London dichroscope”), the other uses a piece of optical-grade calcite. Both will give you a split image, which shows any pleochroic colors present.

The dichroscope can also help confirm other gemological test results. Occasionally, a refractometer or polariscope will give ambiguous readings. The dichroscope can clarify that information. For example, it is often difficult to tell if you’re seeing anomalous double refraction (ADR) in red and purple stones. If you find pleochroism with the dichroscope, you can be sure the stone is doubly refractive.

The dichroscope can also determine if a doubly refractive gem is uniaxial or biaxial, something that other instruments may not always distinguish.

Faceters can also use a dichroscope to help visualize colors in finished gems cut from pleochroic materials.

This traditional dichroscope utilizes optical-grade calcite. The second photo shows what a dichroic gemstone looks like when viewed through this tool. Photo © Joel E. Arem, PhD, FGA. Used with permission.