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CHELSEA FILTER








Absorption Of Light, Developed jointly in 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 stimulants.

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:

GREEN STONES

Natural EmeraldsUsually appear pinkish or reddish since Emeralds absorb in the yellow/green
Synthetic EmeraldsSimilar reaction, but in most cases, they appear a more intense red
Emerald SimulantsPastes (Glass), Soude Emeralds and Doublets appear green
Demantoid Garnet & Green ZirconSimilar to Emerald but will produce a negative reading on the refractometer (the scale will appear uniformly dark since no total internal reflection occurs)
Green TourmalineGreen
Chrome Green TourmalineRed or Pink
Synthetic Green SpinelGreen
Peridot & Green SapphireGreen

BLUE STONES

Synthetic Dark Blue Spinel or Natural Cobalt Blue SpinelRed
Synthetic Light Blue SpinelOrange
ZirconGreen
Cobalt GlassDeep red
Aquamarine & Blue TopazGreen
SapphireDark green, almost black
Lapis LazuliWeak brownish-red

EXCEPTIONS:

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

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

Natural Blue Spinels appear red.

OTHER USES:

The Chelsea Filter can also be used to separate Natural Green Jadeite from color 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 color enhanced Jadeite remains inert under the filter.

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