Synthetic ruby, by definition means one made by man, whereas a natural ruby is found in Nature.
It is as simple as that, but it is usually difficult to tell the difference with a faceted gem, which can be a problem if you are buying such a ruby, or jewelry set with rubies. Synthetic rubies are mass produced by the flame fusion method and large faceted stones (5 to 10 carats) may sell wholesale for a few dollars each.
Natural rubies are found in the gem gravels of Burma, Thailand, Sri Lanka, East Africa and elsewhere. Often they are small water worn "pebbles" of size 3 to 6 mm in diameter, quite transparent red in color, usually associated with blue sapphire and maybe with other gemstones, such as spinel and garnet. Even the smallest sizes, say down to 3 mm, are faceted by local lapidaries to give cut stones of 0.1 to 0.2 carats since they command a premium price as a natural ruby gemstone. A top quality natural ruby of 1 to 2 carat size is a rarity and may sell for several thousand dollars. Why is this so?
The gemstone buyer still values the natural gemstone as a symbol of wealth. If you are wealthy and give your girl friend jewelry having synthetic rubies you would be considered a cheapskate, and so future relations may not be enhanced. Therefore you pay up to own a natural ruby, knowing it possesses that mystic "rarity" factor, which puts you in good stead.
Rubies were first commercially produced in France by Auguste Verneuil in the 1900s by a process involving dropping powdered alumina through a vertically constructed oxyhydrogen flame. The alumina melts (m.p. 2050 C), together with a dash of chromium oxide to get a red color, and at a lower level this melt cools and crystallizes as a single crystal "boule" of ruby. Growth rate is about one inch per hour. Ruby laser rods up to 8 inches long and one inch in diameter were initially made by this method in the 1960’s. This is called the "flame fusion process" and now accounts for hundreds of tons of synthetic corundum per year, which is used in cheap jewelry and for industrial purposes.
Today, small amounts of synthetic ruby are made by other processes developed for special purposes. These crystals occasionally are encountered as gemstones, which has made their identification as synthetics a problem for the gemologist. The "crystal pulling" of ruby from an alumina melt was devised to grow high quality laser rods. Growth from a molten flux at 800 to 1000 degrees C takes months to grow sizable crystals. The "hydrothermal method" operating at 400 to 600 degrees C and ca. 1000 bars water pressure, similarly has slow crystal growth rates of 1 or 2 mm per week. Each process produces ruby crystals having subtle distinguishing features known only to the expert.
Synthetic rubies made by what ever method all have the same chemical composition Al2 O3 as natural rubies (unlike synthetic spinel) with some minor variations in trace amounts of coloring metal ions, particularly chromium. So both synthetic and natural ruby have a refractive index of 1.76 to 1.77, double refraction of 0.008, specific gravity of 3.99, or very close to these figures. This means that regular RI and SG measurements made by gemologists will not distinguish between synthetic and natural ruby. Also, ruby’s distinctive absorption spectrum due to the presence of chromium, with its strong doublet line at 694 nm in the red and other lines in the blue part of the spectrum, are essentially the same in both.
So, how do we tell the difference? We have to resort to other minor features that take some experience to master. This involves subtle optical measurements and internal observation of microscopic inclusions, structures and color variation, or lack thereof.
Natural rubies are not as pure chemically as synthetic rubies which as far as I am aware the latter have only chromium as a minor component. A typical natural ruby, besides containing a little chromium, may also have traces of iron and titanium, which cause the blue color in sapphire. The consequences of this may manifest themselves in slight differences in color, in dichroism and fluorescence response.
Synthetic ruby glows very strongly red when bathed in blue LED light or long wave UV, in fact you may take this as the maximum fluorescence response for ruby. Natural ruby has a variable response, from strong to weak red fluorescence depending on the amount of iron present, which has a quenching effect. Burmese (Mogok) rubies in general glow more strongly than those from Thailand, and with some rubies from East Africa the response is dim. Hence comparing the fluorescence of a ruby of unknown origin with that of a synthetic ruby can give an indication of origin.
Ruby is noticeably dichroic (use a dichroscope) showing the more desired crimson (often with a purplish tint) when viewed down the optic axis (pure o-ray) where as when viewed at right angles (across the prism) you get a combination of the red o-ray and the yellowish red, or orange of the e-ray. A properly cut valuable natural ruby will preferably have the table facet cut perpendicular to the optic axis and thus show no dichroism in this direction, only the desired crimson color being present. On the other hand, synthetic rubies are rarely orientated to get the best color and often show dichroism through the table facet which is a give away, or show more yellowish than purplish tints of red. Optical orientation for cutting synthetic ruby seems to be random.
Synthetic rubies are usually free of visible flaws or inclusions and are more perfect than natural stones. A flawless ruby of 5 or more carats is sure to be synthetic unless you move in high society. Internal features are examined by means of a loupe and binocular microscope, often with the loose gem immersed in a liquid of high refractive index. Distinctive features observed in flame fusion synthetics, which are most frequently encountered, may include curved structure lines and very tiny gas bubbles, but often they are clean to loupe examination.
In natural stones color zoning if present is straight and parallel to the prism of the original crystal. They often contain tiny inclusions of other minerals such as rutile needles or "silk", zircon, spinel, mica etc., and have veils of tiny fluid inclusions, which are not found in flame fusion or melt grown synthetic rubies. Heat treated rubies are common in the market. They can have incipient cracks, and surface fissures filled with lead glass (fracture filled) to improve color and clarity, but they sell at discounted prices.
Some flux grown rubies are more deceptive because they can have veils of tiny flux filled inclusions. These can be identified as a flux glass, rather than water, by their high refractive index and definition.
Hydrothermally grown rubies are even more deceptive but fortunately there not many around, they being a rare collector’s item. They can have similar inclusions as natural ruby and may contain veils of two phase inclusions (water plus a gas bubble), the liquid having very low refractive index compared to the host ruby. Color zoning may be straight as in natural ruby, and parallel to the plane of the seed plate.
In conclusion, when buying a ruby that costs a significant amount from a jeweler or dealer always get with it a Certificate of Authenticity saying it is a natural stone, as determined by specified testing done by a GIA or FGA qualified gemologist. There are several Internet dealers that specialize in selling synthetic gemstones and give the method of synthesis. Thus the gem collector and gemologist can for minor cost study the various types of synthetic ruby, and with the knowledge acquired can more reliably identify the natural counterpart when encountered.
(1) "Gem Testing" by B.W.Anderson, 1971, Butterworth, London
(2) "Gems" by Robert Webster, 1962, Vol 1. Butterworth, London
(3) "Identifying man-made gems" by Michael O’Donoghue, 1983, NAG Press Ltd, London
Some Internet dealers that sell natural rubies are:
www.gemstones.world-market-online.com/Gemstones/Ruby (also synthetic)
Some Internet dealers that sell synthetic rubies are: