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Dear Ettiene, Gemstone colour enhancement There are a number of methods
of changing a gem material's colour. The primary methods are heat,
staining and irradiation.
Heat is the most important method. Up to 90% of corundum on the
current market is supposed to have been heat treated as have a number
of other gem materials (tanzanite, citrine). It is applied in
carefully controlled kilns or primitive ovens in the source countries.
Even heating is essential. These days glass infilling is also the
norm.
Examples include: Brown zircon may be turned to white (transparent) by
heating. These may fade with exposure to light (caution when using
Ultraviolet light equipment) and turn a yellowish brown. Reheating
will remedy this. All white zircon is heat treated.
Brown carnelian turns orange-red.
Aquamarine may go from green-blue to blue.
Rubies may lose a purplish tint.
Sapphires may have their colour deepened or reduced depending on the
material used.
Iolite may be turned a deep blue.
Unethical/fraudulent methods (when undisclosed) include: (induced,
usually surface colourings) Synthetic corundum is treated with
(titanium) to diffuse into the surface to increase blue, induce or
increase asterism or heated to reduce silk or colour zoning. These
treatments are also applied to natural stones. Some verneuil
synthetics are also fractured and the fractures healed to produce
naturalistic 'fingerprint' inclusions and stained with iron oxide
which penetrates the stone and adds to the naturalistic effect.
Diffusion treatment (corundum) may be identified by immersion in
methylene iodide where facet edges and girdles may be strongly
coloured while the rest may be patchy due to uneven repolishing after
the heating/diffusion procedure. Surfaces may be pock-marked and
girdles have 'scabs' on them. Heat treatment also shows up in the
inclusion scene with 'atoll-like' rings and circular 'lily pad'
inclusions, rounded melted crystals, intensified colour banding,
zoning, natural structures and intensified cloudiness.
Diamond may be coated with various chemicals or even dyes. The
coating used (similar in some cases to that used on camera lenses to
improve colour) may be removed and checked for by boiling the stone in
sulphuric acid. There are reports that techniques are being developed
to allow diamond to be applied as a 'coating' or in a thin layer to
other materials, metals and by inference gemstones. This, if
developed will no doubt introduce a new set of problems to
identification.
Staining Used for softer, porous stones that includes:
Chalcedony-green dyed to represent chrysoprase (see spectrum).
Jadeite stained, may show broad line in red not present in most
natural absorption spectra (except Yunnan jade), colour collects in
fissures and veins at the surface.
Crocidolite, in unnatural colours.
Onyx, various colours, dyes, carbonizing treatment (sugar/acid).
Opal, carbonizing treatment: microscope shows pattern of carbon
spots and a possibly greyish look to the stone.
Turquoise, often dyed, oiled, impregnated with plastics or silica.
Note that opal may also be silica-stabilized or plasticized by various
companies.
Emerald, dyed, most often oiled to hide cracks.
Quartz, crackled and dyed to resemble emerald, a technique which goes
back to the Egyptians. Note that a usual test for dyes is a swab with
acetone or alcohol.
Irradiation This concerns primarily diamonds. Methods include:
(diamonds) Radium: green to black, strongly radioactive, no longer
done. Test is exposure on photographic film or paper, or geiger
counter.
Electromagnetic radiation: (cyclotron), green to black, heated
afterwards to produce yellows to golden brown. A surface treatment,
may show a ring around the girdle according to the direction
irradiated as well as an 'umbrella' on the culet. Test by immersion,
dark outline of the stone, etc. Diagnostic absorption line at 594.0
nm for the yellow and browns.
Neutrons: green to black, yellow and browns with subsequent heat
treatment, colour permeates stone, greens not readily identifiable as
treated. Line at 594.0 nm in brown and yellow.
Electron radiation: (accelerator) Blue to blue-green. Natural type
IIb stones are electroconductive and this supplies the test for the
stone as the treated stones are non-conductive ordinary diamonds.
Beryl, topaz, blues are intensified, beryl blue similar to Maxixe
stones with colour in ordinary ray (unlike the natural with colour in
the extraordinary ray) and they fade on exposure to ultra-violet light
(sunlight).
These colourations are accepted commercially where the stone is sold
as a treated stone and the treatment is permanent so that chipping,
repolishing and wear will not remove the colour. Surface treatments
are usually relatively fragile and are in the main used fraudulently.
Readings in Gem Testing aRe: pp. 99-104, pp. 126-157 Detection of
synthetic, imitation and composite stones, pp. 225-227 Diamond
substitutes, pp. 231-238 Colour inducement in diamonds, pp. 245-253
Synthetic ruby, pp. 259-264 Synthetic sapphire, pp. 270-263
Synthetic emerald
Further reading: If interested in the subject one should read the GIA
and GA journals regularly for new developments. Useful books aRe: Gems
Made by Man, Nassau, Kurt, 1980, Chilton Books, Radnor, Pa. Gemstone
Enhancement, Nassau, Kurt, 1984, Butterworths, London. Identifying
Man-made Gems, O'Donoghue, Michael, 1983, NAG Press, London. (This
last has some inaccuracies and badly written passages but still
contains useful information when read carefully. It has very nice
inclusion photographs).
Gemmology Sites to Visit Lewton-Brain©1997/98
http://www.deepcove.com/cig/ Gemmology World-a superb site for gem
information, now has a discussion bulletin board.
http://www.geology.wisc.edu/~jill/306.html An excellent gemology
course and info site. Don't miss this one if you are into gems.
http://geology.wisc.edu/~jill/keywords.html Jill's hot index to her
site on gems. This site is a must visit for those studying gemmology.
http://geology.wisc.edu/~jill/tables.html And Jills tables-if you are
interested in gemology this whole site (and this page) are just
fantastic.
http://www.ruby-sapphire.com/r-s-bk-quality3.htm
http://www.ruby-sapphire.com/r-s-bk-quality4.htm Exerpts from Richard
Hughes great book on rubies and saphires.
http://www.uio.no:80/~hansjb/misc.htm A gigantic listing of geology
and mineralogy sites.
http://www.dendritics.com/ An interesting site for its gemology links
and information.
http://www.bel-cg.com/index.htm BijouX Extraordinaire. a comercial
site with some good consumer articles on gemology and more developing.
http://www.xs4all.nl/~mineral/chem.html An interesting chemical and
crystallography site, good periodic table of the elements.
http://www.deepcove.com/cig/mlynek.html A really good consumer
oriented FAQ on diamonds. Gemologists should read it too.
http://www.ags.org/info/jwlinfo.html American Gem Society information
page, good historical and some gemological info.
http://www.diamonds.net/websites/ A really huge set of comercial and
educational links for diamonds, the jewelery trade and more, check out
the rest of the site too.
http://www.gia.org/ The Gemological Institute of America site-very
comercial but interesting.
http://www.gemzone.com Gem Zone, a gems, tools and equipment seller.
http://www.gemtec.com/inclusn1.html A great collection of
photomicrographs of gem (emerald inclusions)
http://mineral.galleries.com/default.htm A really nicely done
gemmology on-line resource, a search engine and more for gem materials
and their attributes.
http://www.rockhounds.com/rockshop/ Bobs rock shop, look around, Gem
crystal systems info-good stuff
http://www.diamondcutters.com/tutorial.html A tutorial on diamonds and
diamond grading with pictures.
http://www.wehug.com/diamondalert1.html An interesting page on new US
guidelines that allow jewelers to sell laser drilled and filled
diamonds to customers without informing them of the treatment. Wow.
See the rest of the site.
http://www.vipxlnet.com/lapilia/accueila.htm A very nice gemstones
site including innovative facetted gem designs and instructions for
cutting them. identification indicator tables and pictures of examples
(for sale).
http://www.agt-gems.com/AGTbook.html Lots of information about
gemstones and their 'planetory gemology' and astrological meanings.
Check out the 'Modern Gemology' section.
http://www.gemstones.com/amulet_bin/menu/gems/index.html?sLoYmqbWq5a
Gem lore and ascribed attributes for healing. Lots more of interest
to gem people too.
http://www.octagamm.com/gemcache/gemframe.htm Gem 'Pharmacopea',
listing of gems, ascribed meanings and attributes.
http://www.eyesoftime.com/crys/index.htm A good article on crystals,
history of gemology and gemology in general. The 'Eyes of Time' site.
http://www.wtinet.com/gemtest/refractometer.htm A page explaining how
to use a refractometer with refractive indices for gems. More similar
information on this site. (they are a instrument company)
http://www.rahul.net/infodyn/rockhounds/rockhounds.html A very large
set of links for rockhounds and related fields.
http://www.g-s.ch/Service/Schmucklexikon/Schmuck_R.htm German
jewelers/gemstones online interlinked dictionary/glossary.
Charles Lewton-Brain/Brain Press Box 1624, Ste M, Calgary, Alberta,
T2P 2L7, Canada Tel: 403-263-3955 Fax: 403-283-9053 Email:
brainnet AT cadvision.com
Metals info download web site:
http://www.ganoksin.com/borisat/tree.cgi Book and Video descriptions:
http://www.ganoksin.com/kosana/brain/brain.htm
Gallery page at: http://www.ganoksin.com/brain/gallery.htm
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