A synthetic gemstone has the same properties
as a natural stone; that is, all the physical, chemical, and optical characteristic
are the same. The only difference is that synthetic gemstones are created
in a laboratory. So remember they are not a minerals, since they are not
made by nature. If a substance is made to resemble a gemstone, but
does not share its characteristics, it is called a simulant.
Many people get confused and unscrupulous dealers sell simulants under the name synthetic. Simulants can be almost as difficult to detect as synthetic gemstones, but unlike synthetics specialized detectors can reveal differences in the physical properties and detect the simulant; this would not work for a synthetic. One of the simplest tools for detecting simulants is a refractometer, which measure a property related to the speed of light in the stone called the refractive index of the stone. Glass is the most common simulant and will have a different
There are synthetic versions of many popular gemstones and many of them have been around for a long time. Older synthetics are simple for gemologists to detect; they were often too good (no flaws). Some modern synthetic gemstones are more difficult to detect. It takes experience and some suspicion. A jeweler or gemologist can usually detect synthetics by their unusual inclusions or growth features that are not found in a natural stones. As well, certain characteristics of some synthetic stones, such as most early synthetic diamonds made by General Electric (GE) behaving as semiconductors, are rare in natural stones. Thus be skeptical of something that seems too good or unusual to run across on an average day.
Some of the most important synthetic stones are mentioned below. Mostly these are produced because they are inexpensive and replace a natural stone. As an example, synthetic ruby is used as a gemstone, but synthetic quartz, which does have valuable industrial applications, is seldom used as a gemstone because natural quartz is abundant and inexpensive. Some synthetic stones are quite valuable for example synthetic diamonds are selling for about 2/3 the cost of natural ones of the same quality.
Because of limitations in production and the demand of the consumer for natural products, synthetics are never as valuable as natural stones. One example of a limitation is that very large synthetic diamonds have not yet be grown in gem quality and thus do not yet compete for this market. But even this will change soon!
Commonly synthesized stones include: alexanderite, corundum, diamond, emerald, opal, and spinel. Several can be made by the same process. The major synthetic products and processes are discussed below.
Synthetic Corundum (and gems created by similar methodology)
Verneuil method (flame fusion)--this method perfected by August Victor Lewis Verneuil uses flame fusion and powdered aluminum oxide (Al2O3) to grow an elongate rounded mass of corundum called a boule. Powdered Al2O3 and a coloring agent such as chromium are fused as they are dropped through a hydrogen and oxygen flame in a furnace. The apparatus used is similar to a blow torch, and the powder is melted as it falls through the flame.
The corundum is grown on a seed or rod of synthetic corundum. Boules up to several hundred carats (about an inch wide by 3 inches long) are produced. Synthetic corundum gem material can have some properties not seen in natural stones which allow for moderately easy detection.
Other material made by this method includes: rutile, spinel, and strontium titanate (that was used as a diamond simulant). The verneuil method has been used by many companies to create gems, but perhaps the most important advancements of this method were achieved by the Linde Air Company, Chicago. They invented methods to control the growth direction of the boule's crystallography and also invented the synthetic star sapphire in 1947.
Hydrothermal method--this method uses a high temperature and pressure vessel that acts like a pressure cooker. Al2O3 is melted in a hotter part of the vessel's chamber and reprecipitates/crystallizes on seed crystals in a cooler part of the chamber. Crystals are formed rather than a boule and the material appears more natural. A platinum wire sticking through the crystal gives it away, but when the synthetic material is cut away from the wire and the seed crystal, these gems may be difficult to detect.
A lack of natural inclusions and perhaps flakes of metal
from the walls of the growth chamber of the vessel may indicate a synthetic
gem.
Flux growth--This method is popular for both ruby
and emerald. Seed crystals are lowered into a crucible (vessel containing
the substance of interest) that contains feed of the same composition as
the mineral to be grown plus flux. Flux is a substance used to lower
the melting point of the feed so that crystals can be grown at temperatures
below the normal melting point of the gem material. Flux is also
a name used for applications in welding and soldering, but the meaning
of the word is not quite the same as in gem making.
Crystal Pulling (Czochralski Process)--This method is used considerably in the production of ruby and other materials for lasers; it is also used for gems including: corundum, alexanderite, and the diamond simulants GGG & YAG, which have no natural counter parts.
A melt (hot liquid) of the correct composition is prepared
in a crucible that is usually heated using radio frequency methods and
a seed crystal is lowered into it. The temperature is reduced to
just the below the melting point of the seed and the seed it then rotated
and slowly lifted out of the melt. A boule-like rod is created as
the seed is pulled out. The method creates large, very pure crystals
that are used in industrial applications as well as gems.
Synthetic Diamond
High pressure and high temperature method (HPHT)--This method is the classic method first used by GE in 1954 to create industrial diamonds and led to the creation of gem grade diamonds by around 1970. Russian scientists (Kiev Synthetic Diamond Research Institute, 1967) made the first synthetic gem diamonds. Several gem diamonds were produced; however, they were not an economic success. However, industrial diamonds used in drill bits and saws have been profitably produced by these methods for the last several decades.
These diamonds are grown under similar conditions to natural stones, temperatures of around 1,500-3,000oC and pressure of between 852,365 to1,500,000 pounds per square inch (58,000-100,000 atmospheres).
Chemical vapor deposition method (CVD)--This method is often used for depositing a thin layer of diamond on cutting tools or other objects that need a hard surface. The term vapor implies a gaseous phase and the gas used is often methane (CH4). There is no need for high pressure, but temperatures are moderately high, between 750-1,000oC. Not all substances can be coated; it depends on their resistance to melting and whether they form a carbide coating (good) or absorb the carbon (bad). Most importantly, it is possible to grow a diamond on an existing diamond substrate and the rates of growth may be quite high. A gem quality 3-carat diamond may be grown in a few days. Because a seed diamond can be used to orient the growth, the diamond produced may be grown with a controlled direction that allows for best cutting properties.
Cubic Zirconium (CZ)
Skull melting method--this method was invented to produce cubic zirconium (CZ) from zirconium oxide powder. Zirconium has such a high melting point (2,730oC) that when melted, it cannot be contained in a crucible or other simple containment vessel. The answer, simplicity itself, is to heat the zirconium powder from the inside and use the remaining zirconium powder surrounding the hot part as a containment vessel, with the tightly packed unmelted powder acting as the containment walls. This is accomplished by wrapping a radio frequency generator around the mass of powder. The inside of the mass is heated and melts. Upon cooling the interior portion is clear and inclusion-free. CZ is perhaps the best of the diamond simulants, because it is made by a simple method, it is relatively inexpensive.
Opal
Opals is a hydrous (water-containing) form of silicon dioxide. Opal's synthesis is achieved by creating uniformly sized, microscopic spheres of silicon dioxide that are packed closely together, forming a solid mass. The microscopic spheres act like a diffraction grating breaking up light into many colors. The exact methods used are proprietary and difficult to achieve. However, it is clear that spheres of silicon dioxide are created in an aqueous solution and settle slowly to the bottom of a vessel. Once a mass of these spheres is created, heating or other methods are used to consolidate the spheres into a mass.
Both white and black opal are made. The most prominent company making synthetic opal is Gilson Opal ( after Pierre Gilson's product that first appeared on the market in 1974).