In gemmology, a Chelsea filter is a dichromatic

Dichroism

Dichroism has two related but distinct meanings in optics. A dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths , or one in which light rays having different polarizations are absorbed by different amounts.The original meaning of...

 optical filter used for identifying coloured stones.

History

The "chelsea" filter was originally devised by Anderson and Payne in 1934 of the Gem testing Laboratory of the London Chamber of Commerce & Industry

London Chamber of Commerce & Industry

The London Chamber of Commerce & Industry was established on 25 July 1881 at Mansion House in the City of London with 130 members. The London Chamber represented the London Metropolis and not just the City of London. Many of the original 130 members still play a role in London Chamber today...

. The filter was devised with the collaboration of gemmology students of the Chelsea College of Science and Technology

Chelsea College of Science and Technology

Chelsea College of Science and Technology was established as a College of Advanced Technology on a single site on the corner of Manresa Road and King's Road, Chelsea, London as part of the University of London in 1966 and was granted its Royal Charter in 1971....

 where Basil Anderson was an instructor for the Gemmological Association of Great Britain

Gemmological Association of Great Britain

The Gemmological Association of Great Britain, also known as Gem-A, is an international gemmology education and qualifications body based in the UK.-History:...

. Since this filter allows transmission of both deep-red wavelengths around 690 nanometre

Nanometre

A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

s and yellow-green wavelengths, around 570 nanometres, that matched emerald

Emerald

Emerald is a variety of the mineral beryl colored green by trace amounts of chromium and sometimes vanadium. Beryl has a hardness of 7.5–8 on the 10 point Mohs scale of mineral hardness...

's emission and absorption characteristics, this filter was initially recommended to assist the discrimination between natural emerald and its simulants such as green glass, tourmaline

Tourmaline

Tourmaline is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gem comes in a wide variety of colors...

, peridot

Peridot

-Chemistry:The chemical composition of peridot is 2SiO4, with Mg in greater quantities than Fe.-Etymology:The origin of the name "peridot" is uncertain...

, etc. This discrimination is possible because chromium

Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

 (containing iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

) and vanadium

Vanadium

Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...

-free emeralds emit a red fluorescence when illuminated by white light that also has a content of ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 wavelengths.

Synthetic emeralds were commercially introduced around 1940. These produce the same pink-red response as some emeralds through the Chelsea filter. However, although this filter is unable to predictably discriminate between natural and synthetic emerald, it has been subsequently found capable of distinguishing aquamarine, blue topaz, and their blue synthetic spinel

Spinel

Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...

 simulants, because unlike natural gemstones, blue cobalt

Cobalt

Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

-containing synthetic spinels emit a red fluorescence under white light.

Chelsea Colour Filter is a UK Trademark held by The Gemmological Association of Great Britain

Gemmological Association of Great Britain

The Gemmological Association of Great Britain, also known as Gem-A, is an international gemmology education and qualifications body based in the UK.-History:...

(UK Trademark Registration No. 1473951).

Use

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.

External links

• Chelsea Filter - Gemstone Buzz uses, procedure & precautions.
• Instructions for Chelsea Filter
• MineralLab & other type of filters