Chelsea filter (for artificial colour)
Chelsea Filter is an extremely useful device in examining a wide variety of red, blue and green stones.
It was originally intended to rapidly differentiate between genuine Emeralds and the pastes and doublets, which ressemble them, but it is a valuable aid in identifying other colored stones.
To use the filter, place the gem to be examined under strong illumination. Incandescent light will give the best results, because it is rich in red wavelengths
Emeralds containing chromium will display red under the Chelsea filter, while other green gems such as green sapphire, green tourmaline and peridot will appear green.
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