Gem Enhancment and Imitations
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.
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
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 This,
if developed will no doubt introduce a new set of problems to
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
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.
Note I’ve seen ruby beads stained to improve the color.
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
(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.
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.
(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.
Laser Drilling This concerns diamonds, and the removal os carbon spots
by drilling from behind or the girdle. Needs to be disclosed, and are
sometimes filled with glasses.
Infilling Originally done with glasses, now also with plastics, fills
cracks, firstly with diamonds but now widespread with rubies and
Suggested readings in Gem Testing (Anderson) and Gem Identification
(Liddicoat), get the latest edition of each.
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
when read carefully. It has very nice inclus
Charles Lewton-Brain/Brain Press
Box 1624, Ste M, Calgary, Alberta, T2P 2L7, Canada
Tel: 403-263-3955 Fax: 403-283-9053 Email: @Charles_Lewton-Brain
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