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.
- 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.
- 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|
|11.||Blue Topaz||Blue Green|