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Absorption of Light

Chelsea Filter

Developed jointly in the 1934 by the Gem Testing Laboratory of the London Chamber of Commerce and the Chelsea College of Science and Technology, the Chelsea Filter was primarily used to distinguish Emeralds from its many simulants.

The filter consists of a combination of two gelatine filters that transmit only deep red and yellow/green light. This combination was chosen since Emeralds transmit light in the deep red but absorb light in the yellow/green.

The best results are obtained when stones are examined under a strong electric light (not fluorescent light). By holding the filter close to the eye with the stone(s) receiving as much light as possible, the following reactions can be observed:


Natural Emeralds Usually appear pinkish or reddish since Emeralds absorb in the yellow/green
Synthetic Emeralds Similar reaction but in most cases they appear a more intense red
Emerald Simulants Pastes (Glass), Soude Emeralds and Doublets appear green
Demantoid Garnet & Green Zircon Similar to Emerald but will produce a negative reading on the refractometer (the scale will appear uniformly dark since no total internal reflection occurs)
Green Tourmaline Green
Chrome Green Tourmaline Red or Pink
Synthetic Green Spinel Green
Peridot & Green Sapphire Green


Synthetic Dark Blue Spinel or Natural Cobalt Blue Spinel Red
Synthetic Light Blue Spinel Orange
Zircon Green
Cobalt Glass Deep red
Aquamarine & Blue Topaz Green
Sapphire Dark green, almost black
Lapis Lazuli Weak brownish-red


Not all Emeralds will appear pinkish or reddish through the filter.

Blue Sapphires which show a purple colour change when viewed in artificial light usually appear red.

Natural Blue Spinels appear red.


The Chelsea Filter can also be used to separate Natural Green Jadeite from colour enhanced Jadeite.

If the stone appears red under the filter this is positive proof that the stone has been treated.

If the stone appears green, further tests should be carried out since some colour enhanced Jadeite remain inert under the filter.