Synthetic Gemstones
What Are They?
A substance which has been produced artificially, and which has the same chemical composition, crystal structure and physical properties as its natural counterpart.
There are also man-made stones which do not have a natural counterpart and simulants which may resemble the appearance of a natural stone but differ greatly in all other aspects.
The French chemist Fremy produced the first commercial synthetic gemstones in 1877. These were small ruby crystals and were grown by fusing together a mixture containing alumina in a clay crucible, the process taking some eight days. These were termed "Reconstructed Rubies". In 1885, larger "Synthetic" rubies made their appearance. At first, it was thought that these larger rubies were also products of the earlier fusion method used in 1877. More recent analysis, provides proof that these were in fact products of an early form of "Flame fusion" using alumina powder rather than fragments of ruby.
Today, synthetic gemstones are produced by a variety of methods including:-
- The Verneuil Flame Fusion Process.
- The Czochralski Pulling Method.
- The Bridgeman-Stockbarger Process.
- The "Skull" crucible method.
- The Hydrothermal process.
- The Flux melt process.
Distinguishing Synthetic from Natural Gemstones
Because of the large difference in price between natural and synthetic gemstones, it is important for the gemmologist to be able to clearly distinguish between the two. Gemstone simulants are relatively easy to identify because their physical, chemical and optical properties bear no resemblance to the stone being imitated. Synthetics, pose a more serious problem because in most cases they are identical to their natural counterparts. A gemmologist, relies heavily on the ability to discern even the slightest variances between the two to accurately identify one from the other.
Synthetic Corundum
- Presence of curved striae in Verneuil synthetics. These appear as gramophone lines on a record and are caused by successive layers of growth.
- Presence of bubbles, either round, elliptical or flask shaped sometimes having a tail like a tadpole.
- Absence of "silk" in synthetics. Silk is a whitish sheen seen in some corundums and is due to the presence of vast numbers of microscopically small channels reflecting light.
- Curved colour zones seen when immersed in Methylene Iodide caused by the successive layers during the growing of the boule & imperfections in the mixing of the constituents.
- Variations seen in the absorption spectra. Natural blue and green sapphires have 3 absorption bands in the blue whilst their synthetic counterparts do not.
- Absence of natural inclusions (i.e Spinel crystals in Burmese Rubies or Zircon crystals in Sri Lankan Sapphires)
- Dichroism seen through the table in synthetics rather than through the girdle in naturals.
- Colour is unnatural looking.
- Presence of dark unfused blotches of foreign matter caused by colouring oxides.
- Variations in fluorescence. Natural yellow sapphires fluoresce apricot-yellow while synthetic yellows do not and synthetic blue and green sapphires fluoresce under S.W ultra-violet light while naturals (except some Sri Lankan naturals) do not.
- Variations in transparency to U.V light. Verneuil rubies are more transparent to S.W than naturals.
- Star effect seen in both verneuil synthetic star rubies and sapphires appears as if it has been painted on.
- The above are seldom seen with a perfect star and ideal colour.
- The above always have a neatly ground off base whereas naturals have a lumpy base to retain weight.
Chatham Rubies ( Flux-melt )
- Fingerprint inclusions ( Liquid feathers )
- Net-like inclusions with dense and narrow meshes.
- Disseminated platinum platelets.
- Feathers.
- Chrysoberyl inclusions.
Kashan Rubies ( Flux-melt )
- Flux inclusions that consist of cryolite.
- Gossamer-fine veils of flux inclusions.
- Typical fog or cloudlike inclusions.
- Rain or dot like inclusions.
- Comet tails.
- Paint-like inclusions.
Chatham Sapphires ( Flux-melt )
- Twisted-veil like inclusions, fingerprint inclusions or platinum inclusions.
- Overall yellow with dark blue and greenish patches under L.W Ultra-violet light. Overall blue under S.W.
Synthetic Spinels
- Bubbles less common.
- No curved striae except in small red synthetics.
- Normally used to imitate other stones.
- Synthetic blues show pink or red under the Chelsea Filter.
- Higher S.G of 3.63 to 3.64 and R.I of 1.727 due to excess of alumina used in production.
- "Tabby extinction" or "Anomalous Birefringence" under crossed polars due to internal strain.
- Singly refractive.
- No dichroism.
- Variations in the absorption spectra ( i.e Synthetic blues have three broad absorption bands due to cobalt and synthetic reds have organ pipe spectra )
- Variations in fluorescence ( Natural blues are inert under L.W due to iron while synthetic blues appear red)
Synthetic Emeralds ( General )
- Lower S.G's in synthetics ( 2.65 compared to 2.67 - 2.70 )
- Lower R.I of 1.561-1.564 in synthetics with smaller D.R of .003.
- Variations in fluorescence ( Chathams fluoresce a strong red while naturals are practically inert under S.W)
- Synthetics are more transparent to S.W light.
- Synthetics are a more intense red under the Chelsea Filter.
- In some cases, there are variations in the absorption spectra.
- Natural inclusions ( i.e Mica flakes in Zambian & Pakistan emeralds or Three phase inclusions seen in Columbian emeralds)
Chatham Emeralds ( Flux-melt )
- Lower S.G 2.65
- Lower R.I 1.561 to 1.564 with lower D.R of -.003.
- Distinct red fluorescence under U.V light.
- Rather saturated colour & absorption spectra due to presence of more chromic oxide.
- Wisp of veil-like inclusions resembling thinly dispersed cigarette smoke.
- Phenakite crystals.
- Tabby extinction under cross polars.
- Brighter red under Chelsea Filter.
- Pronounced after-glow under X-Rays.
- More transparent under S.W U.V light.
Gilson Emeralds ( Flux-melt )
- Lower S.G and R.I.
- In some stones, iron oxide had been added to increase the S.G and the R.I. This also suppressed the fluorescence but left a characteristic absorption band at 427 nm and attacked the platinum crucible.
- Wisp veil-like inclusions or phenakite inclusions.
- Tabby extinction under crossed polars.
- Dull red under the Chelsea filter.
- Under L.W they appear a dull mustard to greenish yellow. Under S.W orange.
- Cleaner and better colour.
- More transparent under S.W U.V light.
- Pronounced after-glow under X-Rays.
Linde Emeralds ( Hydrothermal )
- R.I range from 1.571 to 1.578 D.R -.007
- S.G 2.678
- Remarkably clean with constants close to those of a natural.
- Pronounced red fluorescence under both S.W & L.W U.V light.
- Red glow can even be seen under strong white light.
- Bright red under Chelsea filter.
- No veil-like inclusions.
- Nail-like inclusions with the head being phenakite crystals
Lechleitner Emeralds ( Hydrothermal )
- R.I 1.575 to 1.581 D.R -.006
- S.G 2.69
- Uses lower grade beryl as a seed crystal which is in turn coated with synthetic emerald.
- Coating leaves rough surface which must be polished.
- Polishing process leaves either unpolished facets or facets that have been polished right through to the seed crystal underneath.
- Main part of the stone contains inclusions that are common to beryl not emerald.
- Network surface of cracks.
- Immersion in bromoform will reveal surface outer coating.
Synthetic Lapis Lazuli
- Lower S.G of 2.38.
- Softer than naturals.
- Decidedly porous. Immersion in water will cause the synthetic to weigh far more than when it was dry.
- Synthetics react strongly to Hydrochloric acid producing a rotten eggs smell.
- More opaque than naturals.
- Iron pyrites added to synthetics will crumble when a needle is applied.
- Under Chelsea filter the natural will appear a dull red while the synthetic will not.
Synthetic Turquoise
- Absence of the dark irregular brown inclusion or veins of limonite.
- Possess a fine grained structure of round or oval particles in a slightly darker matrix at 50X. Naturals show irregular particles of white material in a pale homogenous blue matrix.
- Weak iron absorption spectra seen in naturals but not in synthetics.
- R.I slightly lower in synthetics but generally hard to detect due to the curved surface.
- Lower S.G of 2.70 compared to 2.80 in naturals. Care should be taken however since the S.G of naturals can vary from 2.60 to 2.80 depending on locality.
Synthetic Alexandrite
- Produced by both the flux-melt and pulling method.
- Veil-like feathers & healed cracks similar to emerald.
- Pulled stones show curved striae & odd gas bubbles with no natural inclusions.
- Fluorescence is unusually strong.
- R.I reported to be .02 lower than naturals.
- Scarcity of good quality natural alexandrites in the marketplace.
Synthetic Opal
- Micro-structure in synthetics is noticeably alike whereas in natural stones it is varied.
- Structure described as resembling "Lizard skin", "Dried leaves" or "Crazed paving".
- Colour patches tend to be even in size and columnar.
- Flash of colour is 3 dimensional rather than 2 dimensionalin naturals.
- Weak fluorescence and no after-glow under L.W in synthetics while naturals not only fluoresce but have a persistent after-glow.
- Synthetics are more porous & stick to the tongue.
- When immersed in chloroform, synthetics rapidly become saturated and loos play of colour. This happens to a far lesser extent in naturals.
Synthetic Quartz
- Colourless quartz is only produced for scientific or industrial purposes.
- Synthetic Citrine or Amethyst is distinguished by transparency to U.V light, absence of colour zoning and natural inclusions.
- Breadcrumb like inclusions in synthetics.
Imitation Stones
Glass
- Flint glass contains large amounts of lead oxide resulting in higher R.I, S.G, lustre, dispersion.
- Can cause stones to tarnish and makes them softer.
- R.I varies from 1.44 to 1.90
- S.G varies from 2.00 to 6.00
- Singly refractive.
- No dichroism.
- Inert under L.W, bright pale green or blue under S.W.
- Abraded facet junctions due to low hardness.
- Bubble inclusions or colour swirls.
- Blue paste shows red under Chelsea filter due to presence of cobalt.
Slocum Stone
- Flow lines and bubbles seen under close examination.
- Transparent when held up to the light.
- R.I of approximately 1.50 ( Crown glass) and higher S.G of 2.45 although constants can vary based on composition.
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