INSTRUCTIONS FOR THE CHELSEA FILTER
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GEM
TESTERS
INTRODUCTION
Some gemstones appear to change colour when
viewed through the Chelsea Colour Filter. This colour change
helps identify many gemstones, distinguishing ruby from paste;
distinguishing garnet (though not all garnets) emerald and
aquamarine from paste. The Chelsea Filter also distinguishes
many manmade stone such as cobalt-blue glass and blue synthetic
spinel from natural blue stones. As with all gem testers care
must be taken since nature does not make gemstones to standard
formulae and variations exist.
HOW TO USE A CHELSEA FILTER
Hold the filter an inch or two from the eye.
Light the stone with a strong light bulb or torch. The stone
may appear to change colour. The filter must be held near
to the eye but there is no need to hold the filter close to
the stone, even items in showcases can be examined providing
they are lit by strong lights.
HOW IT WORKS
White light is made up of all the colours
of the rainbow (a spectrum): red, yellow, green, blue. Colours
are known, technically, as wavelengths of light. A gemstone
will absorb some wavelengths and leave the others free to
reach the eye. It is these which mingle to give the appearance
of colour. Emerald absorbs virtually all the yellow/green
wavelengths; the Chelsea colour filter filters out all but
the yellow/green and deep red wavelengths. Since the yellow/green
has already been absorbed by the emerald only red is left
to pass through the filter.
THE IDENTIFICATION OF EMERALD
Most emerald will appear to change from green
to red when viewed though the filter, paste and other stones
remain green/dark though the filter. The brightness of the
red depends on the amount of chromium in the stone. There
is no chromium in most green stones; in natural emeralds chromium
is present in small quantities; in synthetic emeralds chromium
is present in large quantities. Therefore paste, peridot,
sapphire etc. appear green/dark through the filter; most natural
emeralds will appear red (from dull red to bright red) through
the filter; synthetic emeralds will often appear a brilliant
glowing traffic light red. HOWEVER, some modern synthetic
emeralds appear only a dull red and some natural emeralds
do not appear red at all. The Chelsea Colour filter, originally
designed for testing emeralds, will give a good indication
that you may, or may not, have an emerald but will not give
a simple YES or NO reading, it is more useful for the testing
of other gemstones.
OTHER GEMSTONES - INSTRUCTIONS IN BRIEF
Of GREEN stones the following will
remain green through the filter: Enstatite, paste, peridot,
sapphire, most tourmaline.
The following GREEN stones appear to change
colour when viewed through the filter:
Alexandrite Red in artificial light, brighter
red in daylight.
Zircon Reddish
Demantoid garnet Reddish
Emerald Red - see first page and see below.
Aquamarine A murky grey-green.
There is a synthetic corundum which looks
like Alexandrite, this will appear the same red through the
filter in both artificial light and daylight.
Of RED stones garnets and pastes appear
dark red through the filter; ruby (both natural and synthetic)
and spinel glow red.
Of BLUE stones:
Paste Red (for dark blue pastes)
to green (for light blue pastes)
Spinel Red (for dark blue spinels
to orange (for synthetic light blue spinels)
Zircon Green
Only one natural blue
stone gives distinctive results:
MOST Blue sapphires will appear Blackish
INSTRUCTIONS IN MORE DETAIL
Newcomers to gemmology ask of a gem tester:
Is it 100% reliable? The answer is that no gem tester is "100%
reliable" because the composition of gemstones is not
"100% reliable" - such is the variety in nature.
A Chelsea Filter detects the presence of Chromium and Cobalt.
Chromium produces the very intense reds and greens of ruby
and emerald; cobalt causes the brilliant blue in pastes and
synthetic gemstones. The extent of the colour change depends
on the amount of chromium or cobalt present.
RED STONES
Red stones containing chromium are mostly ruby
(Burma Ruby and synthetic ruby) and red spinel (almost certain
to be natural spinel) - all will glow red.
Thai ruby (Siam ruby) and pyrope garnet contain
chromium too but they also contain iron which prevents the
colour change. Demantoid garnet will appear pinkish.
Red stones which contain no chromium will
show no colour change. They will appear a dark red / blackish
colour (because the filter is so dark, not much can be seen).
These include most garnets and paste.
If you have a parcel of red stones pick out
those which glow a brilliant red (synthetic rubies coloured
by chromium), those which glow slightly red (natural rubies
and natural spinels), those which turn very slightly reddish
or pinkish (MAYBE Thai ruby or demantoid garnet), and those
which remain their own red darkened by the filter to almost
black, pastes and most garnets.
GREEN STONES
The GREEN stone coloured by chromium is beryl,
the best known varieties of which are emerald and aquamarine.
The colour change in emerald is to red and, again, the brilliance
of the red and the extent to which it appears to 'glow' depends
upon how much chromium is in the particular stone and how
much iron is present to dull the effect. Synthetic emeralds
are coloured with chromium with the result that the red glow
seen through the filter can be quite spectacular; the red
glow seen in natural emeralds varies from strong for Colombian
and Russian emeralds to nil for South African and Indian emeralds.
The lack of a colour change to red is an indication that your
stone is not emerald, but not proof; a brilliant glowing traffic
light red is a good indication that you have a synthetic emerald
but not proof.
Three stones which can be confused with emerald
are demantoid garnet, grossular garnet coloured by Chromium
(Savolite) and tourmaline coloured by Chromium - all will
appear red through the Chelsea filter.
How, then, do you distinguish these from
emeralds? Green zircon may appear a pinkish colour through
the filter. Green garnets have a far 'livelier' appearance
than emeralds; emeralds can appear almost 'oily' in lustre,
garnets have a brilliant 'fire'. Both tourmaline and zircon
exhibit strong double refraction: look into the stone with
a good 10X lens and the back facets will appear 'doubled'.
In the case of aquamarine iron dulls the
effect almost completely but there is still a colour change
from a clear blue-green to a muddy grey-green. Beware: some
sapphires also appear 'muddy'.
Alexandrite appears red (see previous page),
pastes, green sapphire, enstatite, peridot and most tourmalines
will remain dark / green.
Stones which usually show only a slight colour
change are green garnet (demantoid), green zircon, and chalcedony
which has been stained green - these will appear reddish or
pinkish.
BLUE STONES
