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The Chelsea filter is the most commonly used filter in gem identification, also known as the “emerald filter”.This filter was first used at Chelsea Industrial School, hence the name “Chelsea filter“. It was originally designed to quickly distinguish emeralds from their imitations.The Chelsea filter consists of a filter that only allows deep red light and yellow-green light to pass through. When observing objects directly through the filter, all objects will appear in only two colors, yellow-green or red.
1.Structure and principle
The structure of the Chelsea filter is relatively simple: the filter is sandwiched between a protective glass sheet and installed in a plastic shell to form a Chelsea filter (Figure 1). It consists of a filter that only allows deep red light and yellow-green light to pass through, through which objects can be directly observed.

Figure 1 Chelsea filter

2.Operation (Figure 2)
（1）Clean the sample;
（2）Place the sample on a white board (on a background that does not reflect light or affect observation);
（3）The light source should be white light, strong light and close to the sample;
（4）Hold the Chelsea filter as close to your eyes as possible and observe the sample at a distance of about 30 cm.

Figure 2 The use of Chelsea filter

3.Application

（1）Helps distinguish natural gemstones from their synthetic counterparts
The Chelsea filter is still effective in identifying emeralds in some cases. For example, if it is a very bright red, it is very likely to be a synthetic product.
（2）Helps to identify dyed jadeite: Jadeite dyed with chromium salts will turn red under the Chelsea filter, while natural jadeite usually remains unchanged.
（3）Helps identify synthetic blue spinel, cobalt glass and synthetic blue quartz. The colors of these three gemstones are all caused by the element cobalt. Under the Chelsea filter, cobalt-colored gemstones appear in varying shades of bright red, while natural blue spinel, blue sapphire, aquamarine and blue topaz appear green or gray-green.
4.Precautions
（1） Use a strong white light source, but weak flashlights and fluorescent lights are not acceptable, and direct sunlight also has poor effects;
（2） The color of gemstones seen under the Chelsea filter is not constant, it is related to the transparency of the gemstone;
（3） The color of each gemstone varies due to differences in the type and amount of colorant.