The International Confederation of Jewellery, Silverware, Diamonds, Pearls and Gemstones (CIBJO) define synthetics, imitations and composite gemstones as follows:
- Synthetic Stones are crystallised or recrystallized products whose manufacture, by which ever method, has been caused completely or partially by man. Their physical and chemical properties and/or their crystal structure essentially correspond to their naturally occurring counterparts.
- Imitation Stones are simulations of natural or synthetic stones, products made entirely or partially by man. These stones imitate the effect, colour and appearance of natural gemstones or synthetic stones without possessing their chemical composition and/or their properties and/or their crystal structure.
- Composite Stones are crystalline or amorphous bodies composed of two or more parts assembled not by nature, but by cementing of the pieces or by other artificial methods. Their components may be either natural gemstones or other minerals, or synthetic stones or a chemical product.
- Reconstructed stones are artificial products, manufactured by melting, bonding, or fusing materials to form a coherent whole. Natural and synthetic chips or waste materials are commonly bonded, as is seen in turquoise, coral etc.
- Synthetic Overgrowth: In 1960, Johann Lechleitner of Austria synthesized a hydrothermal overgrowth emerald on a natural beryl seed. These emeralds were often referred to as Lechleitner emeralds or as sandwich emeralds (in the case of synthetic emerald overgrowth on a colourless seed). Over the years Lechleitner synthesized a number of different overgrowth stones with the finished products having various colours.
- Overgrowth is a process by which feed material is allowed to grow on a seed crystal by a man made process. The seed crystal may be natural or synthetic.
- In this process, a layer by layer growth takes place on the seed crystal. In the case of overgrowth emerald, this results in:
- parallel lines along the length of the stone
- a chess board pattern of fine fingerprint like inclusion or fractures which are observed in the synthetic overgrowth portion only.
- Euclase or phenakite crystals can occur at the junction within the synthetic overgrowth.
- In the case of Lechleitner corundum (overgrowth ruby, blue & yellow sapphire), the internal pattern exhibited:
- wispy veil – like fingerprint inclusions
- flux fingerprint inclusions or sometimes curved growth lines in the synthetic portion
- characteristic square and rectangular appearance of the fingerprints (Lechleitner-design)
- Some examples are:
- Synthetic Pink Corundum over Verneuil Colourless corundum.
- Synthetic Ruby over Verneuil Ruby
- Synthetic Ruby over Natural Corundum
Synthesis of Gemstones: In the earliest recorded attempts to synthesis gems, natural stones were buried in the ground in the hope that they would reproduce or grow large over the years. Experiments by scientists have tried to simulate the natural growth conditions which exist at the time of crystal formation. In recent years, research in crystal growth has led to many important advances in gem synthesis.
There are three major methods which are employed in the commercial productions of synthetic gemstones.
- Flame Fusion Process (Verneuil)
- Flux Fusion Process
- Hydrothermal Process
Some other methods employed to produce sythetic gemstones are:
- Czochralski Process (Crystal Pulling Technique)
- Skull Melting Process
- Gel Growth Process
- Ceramic Techniques (not true sythetics)