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:

1. Synthetic Emerald (Cr) Red
2. Blue Glass (Co) Red
3. Synthetic Blue Spinel (Co) Red
4. Synthetic Blue Quartz (Co) Pink
5. Natural Aquamarine (Fe) Green
6. Natural Zircon (Blue) Green
7. Dyed Green Chalcedony Red to Orange Red
8. Dyed Blue Chalcedony Red to Pink
9. Natural Emerald (Cr/V/Fe) Pink to Red / Green to Yellow Green
10. Demantoid Garnet Reddish
11. Blue Topaz Blue Green