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:
GREEN STONES
| 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 |
BLUE STONES
| 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 |
Exceptions:
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.