Color Filters: Uses and Limitations for Gemology

Color Filters for Gemstone Identification

Color filters are small pieces of plastic that will only allow part of the visible spectrum of light to pass through them. To use one, just shine a bright, full-spectrum light on a gemstone and look at the gem through the filter. Then, compare the color you see to a reference sheet.

Although what you see through a color filter should never be used as proof of a gemstone identification, these tools can help you eliminate some possibilities. Plus, they’re extremely portable, which makes them ideal for examining gems at trade shows.

Chelsea Filter

One of the most popular filters is the Chelsea filter. Originally used on emeralds, it can help you distinguish an emerald lookalike. The Chelsea filter allows yellow-green and deep red light through, so emeralds with chromium as a chromophore glow bright red under a Chelsea filter.

However, emeralds with vanadium as the primary chromophore will appear green. In addition, synthetic emeralds will also glow bright red under the Chelsea filter.

Aside from emerald, other chromium-bearing gems will show bright red under a Chelsea filter. Ruby, jadeite, and alexandrite all contain chromium, and other gems like chrome diopside…