In
gemmology
Gemology or gemmology is the science dealing with natural and artificial gemstone materials. It is a geoscience and a branch of mineralogy. Some jewelers (and many non-jewelers) are academically trained gemologists and are qualified to identify ...
, a Chelsea filter is a
dichromatic
Dichromatic may refer to:
* Dichromacy, a form of color-blindness in which only two light wavelengths are distinguished rather than the usual three
* Dichromatic, describing an optical device which splits light into two parts according to its wave ...
optical filter
An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optic ...
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 and Industry (LCCI) is London’s key hub for the business community, we support members’ businesses through a range of services, advocate on behalf of London’s business community in the most important forums of pol ...
. The filter was devised with the collaboration of gemmology students of the
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 SW3, as part of the University of London in 1966 and was granted ...
where Basil Anderson was an instructor for the
Gemmological Association of Great Britain. Since this filter allows transmission of both deep-red wavelengths around 690
nanometre
330px, Different lengths as in respect to the Molecule">molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer ( American spelling) is a unit of length ...
s and yellow-green wavelengths, around 570 nanometres, that matched
emerald
Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium or sometimes vanadium.Hurlbut, Cornelius S. Jr. and Kammerling, Robert C. (1991) ''Gemology'', John Wiley & Sons, New York, p ...
'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 ( ) is a crystalline silicate mineral group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is a gemstone and can be found in a wide variety of colors.
...
,
peridot
Peridot ( /ˈpɛr.ɪˌdɒt, -ˌdoʊ/ ''PERR-ih-dot, -doh''), sometimes called chrysolite, is a deep yellowish-green transparent variety of olivine. Peridot is one of the few gemstones that only occurs in one color.
Peridot can be found in ...
, etc. This discrimination is possible because
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and h ...
(containing
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
) and
vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pass ...
-free emeralds emit a red fluorescence when illuminated by white light that also has a content of
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiati ...
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
Aquamarine may refer to:
* Aquamarine (color), a shade between green and blue
* Aquamarine (gemstone), a type of blue beryl
Aquamarine may also refer to:
Arts and media
* ''Aquamarine'' (novel), a 2001 novel by Alice Hoffman
** ''Aquamarine' ...
,
blue topaz
Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al Si O( F, OH). It is used as a gemstone in jewelry and other adornments. Common topaz in its natural state is colorless, though trace element impurities can ma ...
, and their blue synthetic
spinel
Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals.
Properties
...
simulants, because unlike natural gemstones, blue
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, ...
-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 (UK Trademark Registration No. 1473951).
Use
Hold the filter an inch or two from the eye. Light the stone with a strong incandescent light bulb or torch, not LED. 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.
Chelsea Filters were also used to help separate aquamarine and natural zircon from synthetic flame-fusion spinel (used extensively in "birthstone" jewelry), as both of the former absorb the red portion of the spectrum and the synthetic spinel did not.
[GemologyOnline.com]
See also
*
Infrared filter
Infrared cut-off filters, sometimes called IR filters or heat-absorbing filters, are designed to reflect or block near-infrared wavelengths while passing visible light. They are often used in devices with bright incandescent light bulbs (such as ...
*
Optical filter
An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optic ...
References
{{Reflist
External links
Chelsea Filter - Gemstone Buzzuses, procedure & precautions.
Gemology
Optical filters