Gemmology Notes - Chapter 14: Imitations, Simulants, and Synthetics
Chapter 14: Imitations, Simulants, and Synthetics
Natural vs. Artificial Materials
Natural materials: Formed by nature without human interference, only fashioned.
Artificial materials: Manufactured, not naturally formed.
Types of Artificial Materials
Synthetics:
Must have a natural counterpart with the same chemical composition (), crystal structure (), and physical properties ().
Gemmologically identical to their natural counterparts.
Imitations/Simulants:
Mimic the appearance (effect, color) of other gem materials.
Do not share the same chemical and physical properties.
Examples:
Natural: Quartz, zircon (imitating diamond).
Synthetic: Synthetic spinel, Cubic Zirconia (CZ) (imitating diamond).
Composite Materials:
Artificially assembled from two or more components.
Give the impression of a single item.
Most common types are doublets and triplets.
Reconstructed Materials:
Artificial products made by bonding or fusing materials.
Made from natural gem material aggregated and re-formed into larger pieces.
Mixtures of natural and artificial materials aggregated to form imitation materials similar to natural materials
Production Methods for Artificial Gem Materials
Verneuil Flame Fusion:
Produces synthetic corundum, synthetic spinel, synthetic rutile, strontium titanate.
Flux-Melt Growth:
Produces synthetic corundum, synthetic spinel, synthetic emerald (and other beryls), synthetic chrysoberyl.
Hydrothermal Growth:
Produces synthetic corundum, synthetic emerald (and other beryls), synthetic quartz.
Czochralski Method (Crystal Pulling):
Produces synthetic corundum, synthetic chrysoberyl, yttrium aluminum garnet (YAG), gadolinium gallium garnet (GGG).
Zone Melting:
Produces synthetic corundum, synthetic chrysoberyl.
Skull Melting:
Produces cubic zirconia (CZ).
HPHT (High Pressure, High Temperature):
Includes BARS and Belt methods for producing synthetic diamond.
Chemical Vapor Deposition (CVD):
Produces synthetic diamond.
Sublimation Process:
Produces synthetic moissanite.
Gel Growth Process:
Produces synthetic opal and artificial plastic opal.
Ceramic Techniques:
Produces synthetic lapis lazuli, synthetic turquoise, synthetic coral.
Melting (with or without annealing):
Produces man-made glass.
Verneuil Flame Fusion Method Details
Materials Produced: Synthetic corundum, synthetic spinel, synthetic rutile, strontium titanate.
Advantages: Low production cost, high growth rate.
Process:
Fine powder of the appropriate composition falls through a hot oxy-hydrogen flame (2000°C for corundum).
The powder melts and solidifies as a single crystal (boule).
One boule takes a few hours to grow.
Corundum Production:
Aluminum oxide powder produces colorless corundum (synthetic white sapphire).
Various colors are achieved by adding small amounts of coloring elements to the feed powder.
Powder becomes semi-molten as it passes through the flame, with complete melting occurring in a thin layer on top of the boule.
Detection and Identification:
Physical properties of synthetic ruby and sapphire are similar to natural ones (RI and SG are the same).
Identification relies on growth features resulting from the manufacturing method, observed with a 10x loupe/microscope under varied lighting.
Gas Bubbles: Trapped gas due to the falling powder having no time for escaping before crystallization.
Appear as tiny dots under magnification due to the large RI difference between the gas and corundum.
May be spherical or distorted (tadpole-shaped).
Occur as isolated individuals or clustered in clouds.
Curved Growth Lines/Color Bands: Slight variations between the layers of the growing crystal, visible as parallel curved structures.
May be hard to see in lighter stones (e.g., yellow).
Heating treatment can diminish the appearance of curved banding and introduce 'fingerprint' inclusions (fine rutile inclusions, giving a hazy appearance).
Ripple or Crack-Like Surface Markings: Fire marks or chatter marks produced during polishing, especially common in corundum polished without care.
Natural vs. Flux-Melt Corundum: Straight/angular structure lines/color bands.
Synthetic Star Corundum (Asterism)
Cause: Inclusions of microscopic, needle-shaped crystals of rutile, arranged in 3 directions at 120 degrees to each other in one plane.
Production:
Verneuil process with the addition of 0.1-0.3% titanium dioxide (TiO2) to the feed powder.
TiO2 is initially held in solid solution within the corundum boule.
Subsequent annealing at around 1300°C for 24 hours causes TiO2 to form oriented, needle-shaped crystals of rutile within the single corundum crystal.
Detection and Identification:
Synthetic Asterism: Brighter color, sharply defined star, very straight rays, regular appearance, potential bubbles and curved growth lines under magnification, geometrically precise cutting style, symmetrical domes of moderate height, and flat, often polished backs.
Natural Asterism: More diffuse color and star, irregular, slightly crooked rays of uneven intensity, less perfect cutting style, irregular dome of variable height, rarely flat back.
Verneuil Synthetic Spinel
Nearly all synthetic spinel is grown by the Verneuil method (same as corundum).
Flat faces may be partly developed on spinel boules.
Distinguishing Features:
Chemical composition and physical properties are slightly different from natural spinel.
Refractive Index (RI): RI = 1.727 (synthetic Verneuil spinel) vs. RI = 1.718 (natural spinel).
Anomalous Extinction/Tabby Extinction: Caused by excess aluminum oxide, creating strain in the synthetic crystal structure (visible under a polariscope).
Gas Bubbles: More complex shapes than in corundum, elongated with converse constrictions ('profilated' bubbles).
Synthetic Spinels as Imitations:
Colorless spinel imitates diamond.
Blue synthetic spinel imitates aquamarine, zircon, or sapphire.
Some are colored blue with cobalt, showing a spectrum with 3 bands in orange, yellow, and green and a color change filter (CCF) reaction of RED.
Flux-Melt Growth
Materials Produced: Synthetic corundum, synthetic spinel, synthetic emerald (and other beryls), synthetic chrysoberyl/alexandrite.
Process:
Elements that make emeralds are dissolved in a heated solution with flux inside a platinum/graphite crucible.
Heated for 3-12 months.
Temperature is lower than in the Verneuil flame fusion process.
Flux-Melt Growth - Emerald:
Small emerald crystals used as seed plates to prevent unwanted nucleation elsewhere in the crucible.
Crystals take 2 months to 1 year to grow, depending on the desired size.
Temperatures around 800 degrees are used.
Long growth times contribute to the relatively high cost of flux-melt synthetics.
Detection and Identification (Emerald):
Lower RI and SG than natural emerald.
Inclusions: Veil or lace-like 'feathers' (usually curved or twisted), small crystals of metal from the crucible (titanium), nail-like inclusions, and flux-filled cavities.
Flux-Melt Growth - Corundum (Chatham rubies, Kashan, Ramaura, Knischka stones):
Oxides of aluminum and coloring elements (e.g., chromium) dissolved in a flux of lead oxide plus a small amount of boron oxide.
Temperature of 1300 degrees.
Growth times vary from 2 months to 8 months, depending on crystal size (faster growth results in more inclusions).
Detection and Identification (Corundum):
Natural and flux-melt corundum have similar physical properties.
Inclusions reveal origin:
Flux-filled cavities in various shapes (feather/fingerprints, 'paint splashes', comet-like structures).
Some flux is orange-colored or white.
Platinum platelets from the crucible.
Hydrothermal Growth
Materials Produced: Synthetic emerald (and other beryls), synthetic quartz, (less commonly) synthetic corundum.
Principle: Imitates crystallization from hydrous (water) solutions at high temperature and pressure, similar to how many minerals form in nature.
Process:
Raw materials are enclosed, partially filled with water, in a sealed metal container (autoclave).
Heated to well above water's normal boiling point (100 degrees).
The sealed container prevents steam from escaping.
At around 200 degrees, the water becomes a hot, high-pressure, dense fluid that can dissolve materials that are normally insoluble.
The autoclave must withstand intense pressure of superheated steam.
Process relies on a temperature difference between the hotter region where feed material dissolves and the slightly cooler region of crystal growth.
Hydrothermal Synthesis - Emerald:
Crystals grow at about 600 degrees and a pressure around 1000 times atmospheric pressure.
Growth rates up to 1/3 of a millimeter per day over 2 weeks.
The autoclave is sealed, so nutrients cannot be replenished/added.
Larger stones are grown by using existing crystals as seeds for the next growth cycle.
Manufacturers: Linde, Biron, Regency, and Russian labs.
Detection and Identification (Emerald):
Inclusions similar to flux-melt emeralds:
Veils,
nail-like cavities capped with crystals,
platinum inclusions from the crucible
Zigzag or chevron-like zones,
'Heat craze'-like structures and growth marks in certain directions.
Tests:
Flux-melt and hydrothermal emeralds appear red under Color Change Filter (CCF).
Some contain iron (Fe) to suppress this fluorescence effect (not red).
Certain natural emeralds from Colombia also show a bright red appearance through CCF, making this test inconclusive.
Hydrothermal Growth - Quartz:
Produced in large quantities, mostly colorless for industrial use.
Some colored quartz is produced for gem purposes, with large quantities in the gem market.
Identification (Quartz): Difficult to distinguish between synthetic and natural quartz due to the lack of diagnostic inclusions.
Synthetic Diamond
First produced in 1951 (small quantities, industrial quality).
Production Methods: Belt and BARS methods, requiring extremely high pressures (approximately 70KB) and temperatures (around 1800 degrees).
Color Variation: Various colors can be produced, including colorless, yellow, orange, brown, and blue.
Further treatment can produce green and pink stones.
Detection and Identification: Requires lab testing.
Inclusions: Metal solvent and dust or 'breadcrumb' inclusions, hour-glass or Maltese cross color zoning.
UV Reaction: Stronger reaction to shortwave UV (SWUV), light reaction to longwave UV (LWUV) (opposite of natural diamonds).
Magnetic Attraction: Attraction to a strong magnet due to minute metallic solvent inclusions trapped during growth.
Note: As the quality of synthetic diamonds improves, magnetic attraction and metallic inclusions become less typical (found mainly in older, lower-quality synthetics).
Imitations and Simulants
Synthetic Opal (Gel Growth)
Play-of-color: The optical effect produced by regular arrays of closely packed silica spheres of equal size.
Marketing: First marketed by Pierre Gilson. Not an exact copy of natural opal; considered a simulant rather than a true synthetic.
Production Stages:
Formation of silica spheres.
Settling of the spheres (approximately 1 year).
Compaction and cementation.
Varieties: White opal, black opal, fire opal (with play-of-color and orange color), blue opals, and pink opals (resembling naturals from Peru).
Detection and Identification:
Columnar structure with polygonal appearance in cross-section (resembling lizard skin).
Patches of color may appear more regular than natural opal.
UV Reaction: Most natural white opals phosphoresce green under LWUV, whereas synthetic opal does not.
Artificial 'Plastic' Opal
Production: Structure of precious opal reproduced using spheres of polystyrene (resin).
Cementation: Achieved by impregnation with plastic.
Detection and Identification:
Plastic has a different RI than the spheres.
Specific Gravity (SG) of plastic is 1.9, while SG of opal is 2.10.
Material is softer than opal, showing scratches and wear.
Ceramic Techniques (Simulants)
Materials Produced: Synthetic lapis lazuli, synthetic turquoise, synthetic coral.
Process: Sintering, where a powdered material is heated just below its melting point and compressed to form a tough, solid mass.
Reconstructed Material: If natural material is powdered down and used, the finished product may be called reconstructed material.
Skull-Melting (Cubic Zirconia - CZ)
Cubic Zirconia (ZrO2): The most convincing diamond simulant produced.
Process: Produced by skull melting technique, which involves microwave heating of materials that melt and recrystallize at extremely high temperatures.
The inner surface of the crucible is formed by the solid, water-cooled, outer zone of the zirconia filling.
Addition of suitable coloring agents allows almost any color to be grown.
Identification:
Most CZ lacks inclusions, though fine bubbles are sometimes seen.
Very inexpensive.
Stabilized colorless CZ is produced in large quantities.
Properties: Hardness = 8.5, , , Dispersion = 0.060
Sublimation Process (Synthetic Moissanite)
Materials: Synthetic diamond, synthetic moissanite (SiC = Silicon Carbide).
Use: Used as a diamond simulant.
Process: Very high-temperature vapor transfer.
Identification:
Almost adamantine luster.
Hardness of 9.25.
Very high dispersion, over double that of diamond.
Frequently has a slight tint of green to brown color.
May have needle-like inclusions.
Double Refraction (DR) can distinguish it from diamond. Requires turning the stone (directional property).
Glass
Use: Used to imitate many transparent, translucent, and opaque gem materials with a variety of colors and effects.
Structure: Amorphous, resulting from rapid cooling of molten glass that prevents the development of a regular, crystalline atomic structure.
Composition:
Crown Glass: Silica with oxides of sodium and calcium; used for mass-produced glass, including molded gemstones and ornaments.
Flint Glass: Silica with oxides of potassium and lead; the lead oxide enhances RI, dispersion, and brilliance while making the glass softer; used for faceted diamond imitations and cut-glass objects.
Various metal oxides can be added to alter glass properties and produce a wide range of colors.
Detection and Identification:
Physical properties vary with chemical composition.
Glass imitations of gemstones typically have:
Hardness = 6 and below
(SR)
There are no natural or artificial SR crystalline gem materials in this RI range.
Glass is also called paste.
Testing:
10x Loupe Observations:
Casting flashes (thin linear ridges at mould edges)
Rounded facet edges
Slight concavity of some or all facets (difficult to obtain a clear reading on a refractometer due to contraction during cooling)
Luster may be lower than that of the imitated gemstone
Scratches, chipping, and conchoidal fractures
Broad, swirl-like striations visible in colorless and colored glasses
Gas bubbles
Polariscope Observations:
May remain dark with a 360-degree turn.
Some stones show varying broad bands of dark and light, appearing as a dark cross in many directions as the stone is rotated (anomalous extinction effect caused by strain within the glass).
Spectrum: Varies based on the coloring elements used.
Plastics
Use: Plastic is used to coat and impregnate gem materials.
Production: Manufactured by inducing the molecules of relatively simple, carbon-based compounds to link together through polymerization, forming immensely long molecules.
Detection and Identification:
Hardness = 1.5 – 3
Sectility = yes, in varying degrees (ability to be easily cut into)
Plastic tends to peel.
Most gems splinter or powder (except ivory and tortoise shell).
Polariscope = isotropic (sometimes anomalous)
Transparent to opaque
Colour = varied
Composite Gem Materials
Doublets or Triplets: Made from 2 or 3 pieces respectively, cemented or fused together to form one stone.
Very deceptive if a natural gem material is used for the crown of a faceted stone or the dome of a cabochon.
Detection:
Inclusions:
Bubbles in the junction layer.
Colored discs or unnatural-looking feathers (caused by deterioration or incomplete adhesion of the cement).
Feathers or other inclusions will end abruptly where they are intersected by the junction plane.
By focusing down through the stone, it may be possible to see a difference in the types of inclusions or growth structure in the two parts of a doublet.
Other Tests:
The spectrum of the composite stone will not correspond with its natural counterparts.
RI and SG will also differ.
The components of a stone and the cement may react differently in UV light.
Opal Composites
Opal Doublets and Opal Triplets
Opal doublet
Thin slices of precious opal, which are too fragile to be used as gemstones,
cemented to a backing of common opal, ironstone matrix, or other black material such as chalcedony, glass or plastic.
Some precious opal occurs as thin seams in a banded brown ironstone matrix.
Stones cut as ‘natural’ doublets from this material has a flat surface and matrix is retained as a backing
Opal Triplets
Cemented on top of a cabochon-cut piece of colourless quartz, glass or plastic,
cemented to the top of a doublet.
This produces a cabochon stone
protecting the thin slice of opal
magnifying the play of colour.
Detection
Viewed from the side, the transparent top of the triplet is usually visible.
When viewed with a lens, the play of colour is seen to lie below rather than at the surface of the stone.
Soudé Gemstones
Imitations: Imitate peridots and emeralds.
Construction: Faceted composite stones with a colored layer sandwiched between two colorless or pale-colored pieces of gem material joined at the girdle.
The gem components can be natural or artificial and may or may not be the same species as the gem being imitated.
Detection:
Immersing the stone in water and viewing it from the side against a white background often reveals the composite structure.
Especially effective when the colored layer is between two pale or colorless pieces.
Avoid using immersion liquids other than water, as they may dissolve the cement or adhesive connecting the parts.
Corundum Doublets
A doublet with a natural green sapphire crown and a synthetic ruby or sapphire base.
Especially deceptive when in a rub-over setting where the join plane at the girdle isn't visible.
Detection:
Natural inclusions in the crown.
Verneuil-type curved growth lines/color banding in the pavilion.
Garnet Topped Doublets (GTD)
A less common composite.
Construction: Red almandine garnet fused (not cemented) to colored glass, then faceted.
Advantages of Garnet:
High luster.
Sharp facet edges.
Possible natural inclusions.
High hardness provides good wearing properties.
Colour Origin: The color of GTD comes from the glass base.
The garnet slice at the top is so thin that its red color is generally masked when the stone is viewed from the top.
Use: Replacement stones in multi-stone jewelry.
Identification:
Garnet is often visible as a red rim around the girdle of an unmounted stone (except in red or pink doublets) when viewed table facet down on white paper in bright light.
The thin garnet top and junction are easily seen if the stone is immersed in water and viewed against a light background.
Under SWUV light, the glass in many GTDs fluoresces bright yellow-green; the garnet tops are inert.
The high luster of the garnet contracts sharply with that of the adjacent glass; this difference is easily seen through a lens.
10x Lens/Microscope:
The layer above the junction plane contains inclusions typical of garnet (crystals and oriented rutile needles).
Bubbles in the junction plane and below it. Those at the junction plane are usually most obvious and numerous and are observed at the same time as the natural inclusions in garnet.
Reconstructed Materials
Pressed Amber
Construction: Combining pieces of amber too small or otherwise unsuitable for individual use.
Softened by heating above 200 degrees.
Squeezed together to create a new piece large enough to work with.
Larger fragments may be combined to form fairly clear blocks.
Powdered fragments are heated and extruded to produce rods of reconstructed amber called ambroid. The softened powder may be passed through a steel sieve in this process.
This reconstruction process has been used for Baltic amber.
Identification:
Some original pieces may be detected as areas of varying clarity bounded by distinct margins.
A flow structure may be present in which blobs of clear amber are arranged in definite lines within a cloudy mass or a distinctive feathered pattern.
Bubbles may be elongated in the direction of flow.
Pressed amber can be almost completely transparent, although more often it is translucent to almost opaque.
Color ranges from very pale yellow to very dark brown.