Chelsea Filter

Chelsea Filter

by Ritika Changrani

Also known as Emerald filter, Chelsea filter is used to identify natural, synthetic and imitated gemstones. Developed by the British Gemmologists Anderson and Payne, this filter was initially developed only for the separation of natural emerald from green beryl. Now it is being used to identify various gemstones.

Chelsea Filter – Gemstone Testing Tool

chelsea filter

Chelsea filters are designed to absorb all lights except red and yellow green which are transmitted through the filter. This causes different gemstones to exhibit different colors when observed through the Chelsea filter. Emeralds, when seen through the filter, appear red or pink in colour, while ruby appears brilliant red. Synthetic blue spinals appear yellowish orange or pink while the gems which are imitations appear green or greenish grey. Cobalt glass imitations of sapphire appear deep red.

How to use a Chelsea Filter

Hold the Chelsea filter close to the eye – may be at a distance of an inch or two from your eye. Provide strong light to the gemstone by lighting with a torch or bulb. Hold the torch or bulb such that the light falls directly down on the stone. When viewed from the filter, the stone may appear to change its colour. Never hold the Chelsea filter close to the stone since it may produce wrong results.

How does a Chelsea Filter Work?

Chelsea Filter works based on the characteristics of the gemstone. The Chelsea filter detects the presence of chromium and cobalt in the gemstone. Chromium, if present in the gemstone, produces red and green colour as in ruby and emerald, and the presence of cobalt gives a blue colour. It is the amount of chromium or cobalt present which gives a dark or light colour.

When white light is passed through a gemstone, it absorbs colours of some wavelength and lets other colours free. The other colours which are not absorbed by the Chelsea filter mix up and give the colour appearance to the eyes. Depending on the appearance of the colour when viewed through the Chelsea filter, it is easy to detect the type of gemstone it is. For example, when white light is passed through the Emerald, emerald absorbs all the yellow-green wavelengths. Chelsea colour filter can filter all the colours except yellow-green and deep red wavelengths. Since yellow-green wavelength is already absorbed, only the red colour passes through the Chelsea filter.

One has to make sure that there is enough light on the stone, else the reaction may not be visible to the eye.

Gemstones identified by Chelsea Filter

Red Stones

Red stones that contain chromium are Burma ruby, synthetic ruby and red spinel. Ruby and spinel when viewed through the Chelsea filter appear red. Demantoid Garnet when examined through the Chelsea filter in presence of light appears pinkish. Though Thai ruby and pyrope garnet contain chromium, there is no colour change observed due to the presence of iron in those stones, which prevents colour change. Red stones like garnets and paste do not contain chromium and hence there is no colour change. When viewed through the Chelsea filter, they appear dark red or black in colour.

Therefore when you are examining a group of red stones, you can easily separate different kinds of gemstones. Those which impart brilliant red are synthetic rubies. Stones which glow slightly red are natural rubies and spinals. Thai and Demantoid garnet may turn pinkish or slightly red. The stones which remain dark red or black are paste and other garnets.

Green Stones

Beryl is a green coloured stone which is coloured by chromium. The different types of beryl are emerald and aquamarine. Natural emeralds have a smaller amount of chromium, while synthetic emeralds have larger amount. Hence the change in natural emeralds is to dull or a little bright red. Synthetic emeralds change their colour to bright red. Some synthetic emeralds produce a dull red colour due to the presence of iron. Some natural emeralds like South African and Indian emeralds do not produce any red glow when viewed through the Chelsea filter. Chelsea filter can just give an idea that the gemstone may or may not be an emerald but the test is not a proof of detecting an emerald.

Some stones can be confused with the emerald. They are Demantoid garnet, green garnets or savolite and tourmaline. All these stones when tested using the Chelsea filter, appear to glow red and are often mistaken to be emeralds.

Green gemstones can be distinguished from each other by some of their characteristics. Emeralds produce an oily luster when seen through a Chelsea filter. Green garnets appear brighter than emeralds. Green zircon appears pinkish in colour and exhibits double refraction. Tourmaline is also a stone which exhibits double refraction. When green zircon and tourmaline are viewed through a 10X lens, the back faces of the stones appear to be doubled. Aquamarine has iron content due to which the colour produced is dull. The colour change in aquamarine is from blue-green to muddy grey-green. Green garnet, green zircon and chalcedony show very little colour change and appear pink or red. Pastes, green sapphire and peridot appear remain dark-green. Alexandrite appears red when seen through the Chelsea filter.

Blue stones

Blue stones contain cobalt which can be detected by the Chelsea filter. The colour change of blue stones when viewed though the Chelsea filter is red in highly doped stones and orange brown or green in light blue stones. The colour change to red when viewed though the Chelsea filter is an indication that the gem stone is synthetic. Sapphires show no colour change and remain dark blue or black.

How to take Care of a Chelsea filter

  • It is recommended not to touch the Chelsea filter as it may leave marks on the glass. A soft dry cloth must be used to remove the dust.
  • Never put the Chelsea filter in water or any liquid.
  • The Chelsea filter must not be allowed to get heated either in direct sunlight or on a radiator.
  • When not in use, the Chelsea filter must be kept closed.

Chelsea filter is an important tool in the field of gemology which can be used to test various stones. Although, the test is not cent percent reliable since the composition of every gemstone is not the same even in the similar type of stones, it is used by a number of gem testers for testing the gemstones.

More Info: Gem Testing Tools