Chelsea Filter

The Chelsea filter is a dichromatic filter which transmits two wave lengths only – in the red region at 690nm and the other in the yellow region at 570nm. The presence of the elements chromium or cobalt in a gemstone makes it appear red when viewed through this filter.

Uses of Chelsea Filter:

• This filter normally indicates the coloring elements present in certain gems.
• Stones coloured by chromium and cobalt will appear red under this filter.
• In some cases it may help to detect dye in certain stones.
• It is generally useful for green and blue stones such as emerald and aquamarine.

Procedure:

• Use strong white light source.
• Light may be transmitted or reflected from the stone.
• Hold the filter close to your eye or to the stone; observe the reaction of the stone.
• It can either be red or green, with secondary shades of pink, brown, yellow, blue or black.

Precautions:

• Stones should be observed from all directions.
• The reaction seen through the stone would depend on size, shape, transparency, cut and colour of the stone.
• Each stone would react differently depending upon the colouring agents present.

• Chelsea Filter is an indicative test. This test in itself is not a confirmatory test of the stone’s identity.
• Presence of iron tends to retard or dampen the red colour in emeralds, ruby etc. and is responsible for the shade variations.
• A synthetic emerald with iron content will tend to behave as some natural emeralds, while a natural emerald with high chromium content will exhibit red like some synthetic emeralds.

Some Common Reactions Under Chelsea Filter: