Learning Library

Chelsea Filter

An Emerald dealer’s oldest companion.

First developed in 1930 by two teachers at the Chelsea College of Science in London, the Chelsea Filter is the most commonly used filter in the gem industry. Also known as the Emerald Filter, its use originally was to detect Emeralds from other gemstones such as Green Sapphires and Tourmalines, as well as man made simulants such as glass.

Until 30 years ago the filter was a very reliable way of telling if your Emerald was the real deal. When viewed through the filter, if your Emerald looked pink to red then you were pretty sure it was real, however today there are many fakes and synthetics that do the same thing and to confuse matters even more, genuine Emeralds are now being discovered in new locations that don’t exhibit pink or red hues through the filter. Although the filter is therefore not as useful as it use to be for Emeralds, it has got plenty of other uses in gemmology and hence its name change. Before we study its uses, lets look at how it works.

The Chelsea filter is probably one of the easiest gem tools of all to use. The filter itself is a green colour and it only allows two colours (two wavelengths) to be seen through it: green and red. Therefore regardless of the gem’s colour, it will only appear either green or red. A vivid purple Amethyst, will appear a Tsavorite green through the filter. In fact the deeper the colour of the gem, the stronger the green or red will appear. A light coloured Emerald will often be more pinkish, whereas a darker vivid Emerald will normally appear red.

To get the most accurate reading from a Chelsea filter, use strong, natural daylight or a strong light bulb. Never use fluorescent light as the lack of red wavelengths will distort your reading. For best use place your gem directly under a lamp so that the light is strong and directly pointing at the table of your gemstone. Bring the filter close to your dominant eye and as you would with most other gem devices close your other eye. With the filter close to your eye, position yourself just five to ten inches away from your gem, but be sure not to block the light source. If you are testing gems that are already set in jewellery, as accent diamonds and yellow gold can often distort the reading, it is best to cut out a small hole in a piece of black paper to put around your gemstone.

So what do you use the filter for? Although its use is best for identifying green gemstones, it also has many other uses. I tend to use the filter on a commercial level differently to how most Gem Collectors would use one. I find the filter invaluable if I am buying big parcels of gemstones. The filter makes it very easy to identify if the seller has mixed one or two synthetic gems in with the parcel. Does this ever happen? For sure! Often when on buying trips in India and Thailand, if I am searching for a particular gem, I might have to see 20 or 30 gem houses before I see the parcel I want to buy and sometimes I have found that the best deals are, as the saying goes, too good to be true. Some unscrupulous suppliers will mix a small percentage of synthetics in with the real gems, that small percentage means they can sell for less and the Chelsea filter is great for spotting the difference. I once found a Tanzanite parcel to contain about 30% synthetic stones and my team quickly marched the seller out of my office!

But what might you use the filter for at home? Here are a few suggestions.
If you have an Emerald and are unsure of the location, if it looks deep red through the filter then it is most likely to have been mined in Siberia or Colombia. Emeralds from Africa, Zambia and India normally do not show any pink or red at all. Why is this? Well, those from Colombia are rich in chromium which is detected by the filter, whereas those from India and Africa tend to be rich in vanadium which does not show up on the filter.

Green Tourmaline’s colour is normally inert and therefore does not change colour when viewed through the filter. The very rare Chrome Tourmaline variety will show a bright red colour if it is genuine as its colour is derived from chromium. If your Chrome Tourmaline does not look red through the filter then it is not Chrome Tourmaline.

Genuine Alexandrite has the ability to look different colours in different lighting conditions. When viewed through the Chelsea filter it will always show a red tone. If it doesn’t then it is most likely synthetic.

If you are given a gem and aren’t sure if it is a Pink Sapphire or a Pink Tourmaline, then place it under the filter. The sapphire will appear more of a red colour and the Tourmaline will look more pinkish.

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Barry Kirley viewing a gem using a Chelsea Filter.


A Chelsea Filter.



Examining another gem.