Many stones appear to change colour when viewed through the filter. Originally designed for detecting synthetic emeralds, nowadays the Chelsea filter is an invaluable aid to identifying many other gemstones.
It can give an indication that you have an emeralds and aquamarines, though not a guarantee, since there is considerable variation in their chemical compositions (e.g. the amount of chromium that gives the green colour and the amount of iron that ‘dampens’ the effect through the filter).
It gives a very clear and spectacular result on blue stones coloured with cobalt. Since cobalt doesn’t colour stones blue in nature, this is a sure-sign that you have a manmade stone (e.g. synthetic spinel) or glass.
With skill, it helps identify dozens of other stones. It’s particularly useful when you have to sort large parcels of stones into ‘probables’ – then you have clues as to which tests to carry out next.
How to use:
Using it is very easy indeed. Shine a light (e.g. our pen-torch) on the stone, hold the filter close to your eye, view any colour change. You don’t even have to hold the stone close to the filter (only the filter has to be close to your eye) so you can examine stones in museum showcases.
Some Familiar Reactions under the Chelsea Filter:
- Synthetic Emerald (Cr) – Red
- Blue Glass (Co) – Red
- Synthetic Blue Spinel (Co) – Red
- Synthetic Blue Quartz (Co) – Pink
- Natural Aquamarine (Fe) – Green
- Natural Zircon (Blue) – Green
- Dyed Green Chalcedony – Red to Orange Red
- Dyed Blue Chalcedony – Red to Pink
- Natural Emerald (Cr/V/Fe) – Pink to Red / Green to Yellow Green
- Dermatoid Garnet – Reddish
- Blue Topaz – Blue Green
