The renowned Chelsea filter is a must have for identifying colored stones. While it was originally intended to rapidly distinguish between genuine Emeralds and the pastes and doublets which resemble them, it also is a valuable aid in identifying other colored stones, as well.
The filter is 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. Just hold the filter close to the eye with the stone or stones (for a number of stones can be examined at one time) 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, soude Emeralds) retain a green appearance.
In rare cases, certain Emeralds, notably those from South Africa, may not show any pinkish reactions. Generally, green synthetic Spinels display green under the filter, though some stones, probably colored by chromium, show red. Synthetic Emeralds react in the same way as genuine stones, though the resident red color is often more brilliant.
A reliable indication of Ruby (both natural and synthetic) is given by a characteristic brilliant red seen through the filter, synthetic Rubies being on the whole more brilliant than the genuine.
Synthetic blue Spinels, which are intended to imitate Aquamarine and Sapphire, appear yellowish-orange or pink when examined, while the stones they are intended to represent appear green or greenish gray.
Cobalt-glass imitations of Sapphire show a deep red, though some other blue glass imitations, colored by iron, do not display any real reaction. Green pastes show green.
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