Chelsea Filter
- Gemstone Testing Tool By: Ritika Changrani
Summary: 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 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
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