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The Chelsea Filter is an essential device in identifying colored stones. While it was originally intended to rapidly differentiate between genuine Emeralds and the pastes and doublets which resemble them, it also is a valuable aid in identifying other colored stones, as well. Designed to transmit only deep red and yellow-green light, and the best results are obtained when stones are examined under a strong electric light. Testing stones is very simple, just hold the filter close to the eye with the stone or stones (stones can be examined simultaneously) receiving as much light as possible. Under these conditions, Emeralds (absorbing in the yellow green) usually appear distinctly red or pinkish in colour, the actual tint varying from merely pinkish in the case of pale stones, to a rich almost ruby red for better coloured specimens. On the other hand, most imitations of the genuine stone (pastes, doublets, soud,Emeralds) retain a green appearance. Chelsea is definitely a helpful tool in identifying certain red, blue and green stones.