The History of the Chelsea Filter
The Chelsea Filter is one of the most useful gemological tools
you will find...and one of the least taught in many gemological
schools in the United States. This filter was developed by two
British gentlemen named Anderson and Payne of the British Gem
Testing Laboratory and named for the place it was first used
at Chelsea University.
The filter was first developed for the separation of natural
emerald from green beryl. At the time only emeralds colored by
chromium were considered to be true emeralds. The emeralds colored
by vanadium were considered to be green beryls. Since the chromium
colored beryl would show the red color through the Chelsea filter
and the vanadium would not, the filter was a diagnostic separations
for natural emerald and natural green beryl.
How it works..
The Chelsea filter works by allowing only certain wavelengths
of yellow-green and red colors to be transmitted through the
filter. As a result, gemstones transmitting these colors will
appear as either green or red, while other gemstones will simply
appear dark. This allows, for instance, chromium based emeralds
to appear red since the red color is transmitted and the deeper
green is not allowed to pass. (only the yellowish-green). Likewise,
in a green cobalt-coated topaz, the red of the cobalt is readily
visible but the green color is not shown. Allowing a quick identification
of a green stone as cobalt treated. The same applies to many
other gemstones that can be viewed with a Chelsea filter. And
while the Chelsea filter reaction can rarely be considered diagnostic,
it is a very good indicator, and can quickly be used for diagnostic
tests when used in conjunction with other gemological tools.
Before we start looking through a Chelsea filter, let's first
learn HOW to use the Chelsea filter. Because proper use is just
as important as having the tool to start with.