Identifying Synthetic Corundum


Throughout history gemstones have been misrepresented; Synthetic gemstone being sold as natural, and treatments go on undisclosed, most of the time this happens unintentionally. A gemstone is purchased and sold several times as it travels down the pipeline to the end consumer. These gemstones could have even been purchased second hand as a recycled item, mixed with parcels and all a buyer has is the word of their vendors.

Synthetics have a place in the jewelry industry so long as they are disclosed properly in the event they are not disclosed they need to be properly identified.

Some gemologists can make a proper identification using classical techniques with enough experience; yet, this is not enough in most cases, especially when dealing with corundum. Too many synthetics have natural looking inclusions because of their growth process or treatments. Trying to identify synthetic corundum with classical gemological techniques alone can be a challenging task, therefore understanding when to send a stone to a gem lab for further testing is essential.

This project will highlight the heterogeneous synthetic processes of corundum. To attain a better understanding of identification it is important to have background knowledge of how these stones are grown. First the history and manufacturing process of synthetic corundum varieties will the reviewed, then identification techniques using classical and practical gemological techniques along with advanced testing will be covered.

Specifically, internal features and chemical composition of synthetic and natural corundum will be detailed, listing results of both natural and synthetic materials to fully understand the differences and confidently making an identification.For the sake of this article “Natural” will refer to unheated, and untreated gemstones unless otherwise stated.

Corundum is only composed of Aluminum Oxide (Al2O3) in its purest form and is called “white sapphire.” The red color of corundum, ruby is caused by chromium while the color in blue sapphire can be caused by iron and titanium (Gemmological Association of Great Britian 2009 D7). Synthetic materials have an essentially identical chemistry, structure and physical properties to its natural counterpart. Synthetic corundum can be grown and separated into two categories: Crystallization from a solution and crystallization from a melt.

Verneuil Flame Fusion (melt)

The French chemist Auguste Verneuil produced the first synthetic rubies in 1902 and this accomplishment was groundbreaking because it resulted in the first commercial use for gemstone synthesis. The flame fusion process is popular due to its low cost and rapid material growth rates. (Koivula, J; Schmetzer, K; Tannous, M; 2000)

When growing corundum, an aluminum oxide power with desired trace elements such as chromium (to produce ruby) for example is shaken onto an oxyhydrogen- flame (about 2000*c). The powder is then liquefied and cools onto a rotating “boule” (without a container) forming ruby. Caution must be practiced whilst cooling the material; if cooled too quickly the material could end up amorphous rather than crystalline. (Gemmological Association of Great Britian 2009, D14,Pg 17-19) The boule usually has a lot of internal stress and oftentimes breaks in two pieces. If the resultant boule does not break it is necessary to split it before cutting begins. (Weldon, R; Intro to synthetic gem materials)

Flame fusion synthetic corundum does have a completely different crystal shape and habit compared to its natural counterpart. See figure below.

Figure 1 Flame fusion method, photo courtesy of Barbara W. Smigel 2012
Figure 1.1 Flame fusion synthetic corundum boule: photo courtesy of GIA

Crystal pulling method (melt)

When gem producing material is melted with a solution into a crucible, it is put in contact with a seed crystal attached to a rotating rod.

The temperature of the melt should not be too high, to ensure the seed crystal does not melt or too low to ensure nucleation does not occur early. After the seed is put into the melt it is raised slowly, and crystallization starts as the crystal is withdrawn. (Weldon, R; Intro to synthetic gem materials) (Gemmological Association of Great Britian 2009 D14 Pg 18-19)

Figure 1.2 Image courtesy of Gem Rocks Auction

Generally free from inclusions, pulled synthetic corundum can easily be identifiable because their crystals are elongated with rounded cross sections. Additionally, when cut they can have curved color banding and elongated gas bubbles (Gemmological Association of Great Britian 2009 D14)

Flux Growth (solution):

Several manufacturers have been successful in the production of flux melt growth synthetic corundum such as Chatham, Douros, Kashan, Knischka and Ramuea although it was not until 1959 that Carrol Chattham introduced flux melt rubies for the very first time. The production of flux grown blue sapphires were more complicated and Mr.Chattham did not introduce these stones into the market until 1975.When molten, flux dissolves materials such as alumina (Al2O3) along with other color causing ingredients (trace elements) in a

platinum crucible. The objective is to make the solution saturated and when this state is reached, the maximum amount of ingredients is dissolved. When the solution cools slightly in a saturated state, nucleation occurs (crystal growth) to create the gem material desired. Flux-melt technique is a great option since the molten flux acts as a type of borax, dissolving substances used for crystal growth that have a higher melting point The flux material is essential for this growth process, without it the amount of heat required to dissolve the gem producing material would cause potential crystals to melt or decompose. (Gemmological Association of Great Britian 2009 D14), (Koivula, J; Schmetzer, K; Tannous, M; 2000)

Figure 1.3 Flux synthetic Chatham crystal, Photo courtesy of Chatham

The flux melt process works like that of nature, producing more natural looking crystals and in most cases producing more “believable”. See figure 1.3

Hydrothermal (solution):

Crystal growth in the hydrothermal process is not as common for corundum but is worth discussing briefly. The hydrothermal process is interesting because it closely resembles a natural way gemstone grow. Gem producing material is sealed in an autoclave (thick metal cylinder) filled with water and a crystal seed. When the water inside the autoclave is heated above its boiling point, the high pressure and dense fluid dissolves the materials which would otherwise be insoluble. During cooling crystal growth occurs on the seed crystal (Gemmological Association of Great Britian 2009 D14)

Hydrothermal synthetics are generally clean, and its mot diagnostic feature is wavy growth structure.

Features of Flame Fusion Synthetic Corundum:

Curved growth patterns and gas bubbles are going to be the most diagnostic feature in Verneuil synthetic corundum, the curved growth represent the way the synthetic crystals grow while being rotated (Gemological Institute of America Gem ID). These curved growth lines are termed “curved striae,” and look like curved lines that jump across facet junctions. Sometimes it can be difficult to detect curved striae and it is necessary to use different lighting techniques with a microscope such as pinpoint lighting, darkfield illumination or diffused lighting whilst rotating the stone in different directions. Consequently, careful positioning is essential. In the direction perpendicular to the gemstone’s optic axis, curved striae can appear straight. In synthetic yellow sapphire or other very light color stones it can be extremely difficult to notice this curved growth pattern. (Elen. S; Fritsch, E. 1999) (Gübelin and Koivula photo atlas of inclusions vol. 3)

Throughout my research, identification of curved striae is hardly an issue except in the case of colorless or yellow sapphire, especially when there is a lack of inclusions (Elen Fritsch, 1999) cross polarized light while immersing the gemstone under test in methylene iodide may accentuate curved striae, or show parallel twinning that intersect at 60 and 120-degree angles. This is known as Sandmeier-plato twinning, this twinning alone is not diagnostic of flame fusion growth (Koivula, J; Schmetzer, K; Tannous, M; 2000). twinning has been seen in both natural and synthetic corundum, so it is important to use plato twinning in conjunction with other indicators to make a proper identification. (Eppler W.F. 1964) (Hughes B. 2017)

Verneuil synthetic corundum with heat induced finger prints can be misleading and easily be mistaken for a natural gemstone especially when the heat treatment diminishes the curved striae. One should be suspicious if a stone being observed has a fracture or fingerprint without having any other noticeable inclusions. (Gübelin and Koivula photo atlas of inclusions vol. 3).Flame fusion corundum can also be quench crackled, this should be easily identifiable by the “honey comb” like appearance shown in Figure 2.1.

It is possible to identify gas bubbles in some older flame fusion material, and they can be present as an individual or cluster of bubbles (Koivula, J; Schmetzer, K; Tannous, M; 2000) (Gemmological Association of Great Britian 2009 D14). It is important not to mix up gas bubbles for natural crystals. One way to be sure of yourself is by using the cross polarized technique. In cross polarized light crystals will show an iridescent color. While gas bubbles due to the difference in RI with the host material and gas bubbles, they usually have higher relief. (Gemological Institute of America Gem Identification)

Figure 2.3 Small gas bubbles with blue color concentrations.

Identifying features of Flux Melt Synthetic Corundum:

Flux fingerprints can resemble natural finger prints, the flux can look white,yellow or orange; upon close examination a gemologist would notice that the flux inclusions are opaque compared to natural liquid filled fingerprints. (Gübelin and Koivula Photoatlas Vol.3)

Flux channels sometimes contain undissolved or grainy residue in the healed channels due to the high viscosity of the flux. (Gemmological Association of Great Britian 2009 D14) While concentrating a fiber optic light under high magnification, or while using bright field illumination, residue can be easy to locate. Flux inclusions twist and turn and so it is important to build enough experience to confidently identify a flux inclusion and not confuse them for natural finger prints. (Gemological Institute of America, Gem Identification) (Henn, U.; Bank H. 1993)

Platinum platelets from the crucible tend to fall into the melt during the crystallization process. These platelets are easily identifiable and alone are diagnostic of synthetic corundum. They have an irregular shape sometimes even triangular with sharp edges (Crowningshield, R. 1970) and (Henn. U; Bank.1993). The platinum is opaque and highly reflective. Using the overhead light on a microscope will show the platinum’s high reflectivity. (Henn U. Schrader, H. 1985)

Figure 3 Kashan flux grown synthetic ruby with translucent to opaque flux inclusions. Photo By Nathan Renfro Courtesy of GIA
Figure 3.1. This reflective platinum platelet observed in the pink synthetic sapphire is a common inclusion in flux -grown synthetic corundum. Photo by Nathan Renfro
Figure 3.3 Platinum crystal in Chatham Synthetic Sapphire, 50x magnification. Photo Courtesy of GIA
Figure 3.4 Angular growth zoning in flux synthetic ruby Photographer E. Billie Hughes Courtesy of Lotus Gemology
Figure 3.5“wispy Veils” in flux synthetic Ruby, this can be misidentified as natural feathers if caution is not taken. Photo Courtesy of Barbara W. Smigel, 2012 3.6

During my research I was able to examine several natural and synthetic samples, below are pictures of some interesting features.

Figure 4 Photo by: G. Joseph Shihadeh
Figure 4.1 Photo by: G. Joseph Shihadeh
Figure 4.2 Photo by: G. Joseph Shihadeh

Figure 4-4.2 are images in darkfield of a 1.99 ct synthetic Chatham flux grown ruby crystal. The flux inclusions resemble fingerprints, yet notice the undulating structure, and undissolved flux residue in the channels; this is proof of synthetic origin (Emmett 2005) (Gübelin and Koivula photo atlas vol. 3)

Figure 5 Photo by: G. Joseph Shihadeh. Reflected light
Figure 5.1 Photo by: G. Joseph Shihadeh. Diffused light
Figure 5.2
Figure 6

Figure 5-5.2 are images I took of a .68 ct round faceted synthetic flux Chatham ruby. Figure 5 shows highly reflective platinum platelets, which are absolute proof of synthetic origin. while Figure 5.1 shows a gloppy orange-red flux. Figure 5.2 shows additional flux.

Figure 6.1 Image by: G. Joseph Shihadeh

When natural ruby is heated to high temperatures, and flux is introduced, healing takes place in the fractures of the ruby. It can be difficult to determine if you are looking at a natural material with flux residue or a flux grown synthetic. Figure 6-6.1 show images of a .41 ct natural ruby from Myanmar with moderate flux residue from high temperature heat treatment.

In figure 6.1 my research has determined that a natural solid inclusion had melted and ruptured, if an inclusion like this is seen we can determine high temperature heat treatment took place; several other treatments follow high temperature heat therefore caution must be practiced when identifying this material.

Figure 7 Image by G. Joseph Shihadeh
Figure 7.1 Image by G. Joseph Shihadeh

In figure 7-7.1 we can see inclusions in a natural 1.20ct faceted blue sapphire.

Small intact prismatic inclusions and natural finger prints, along with two phase incisions indicate natural origin. Looking at as many pre-identified gemstones as possible is the key in fully understanding what is being observed.


One of the most important things to remember is that synthetic corundum has the same gemological properties as natural corundum (Gemmological Association of Great Britian 2009) therefore it is important to outline all of the reactions of synthetic and natural corundum in both short-wave and long-wave UV because these tests can sometimes be diagnostic or used as an indicator in conjunction with other results to make a proper identification. If a gemologist has access to a short-wave UV light it can produce several reactions that can help the identification for varieties of corundum. Sometimes SWUV can accentuate the curved growth zoning typical of flame fusion corundum, (Crowningshield 1970) (Koivula, J; Schmetzer, K; Tannous, M; 2000)

Fluorescence in Ruby

In general, synthetic ruby fluoresces a bright red when exposed to long-wave UV,but because most natural ruby (especially from Myanmar) emit this same sort of bright fluorescence due to the high amount of chromium. Long-wave UV is not diagnostic to make an identification. When testing stones containing higher amounts of Fe such as sapphire or even rubies from Thailand the fluorescence is diminished so again, long-wave UV is not diagnostic. Furthermore, magnification will be one of the more diagnostic tools when not in a laboratory environment.

Fluorescence in Colorless Sapphire

When colorless sapphire is generally clean, it can be almost impossible to make a proper identification (Elen. S, Fritsch E. 1999). Even when inclusions are present I have found it may be extremely difficult to discern whether you are looking at a natural crystal or gas bubbles. When a colorless sapphire emits a chalky like fluorescence under shortwave UV it is diagnostic of synthetic (Elen. S, Fritsch E. 1999) (Crowningshield 1970). When an inert reaction is seen the material under test could be considered natural or synthetic therefore fluorescene is not always conclusive and would need to conduct additional tests. Short wave UV can help in separating growth features like curved striae or angular zoning. (John I. Koivula 2000)(Elen. S, Fritsch E. 1999).

Figure 8 + Figure 8.1

In these photos (figure 8 and figure 8.1) taken by Shane Elen and published by Emanuel Fritsch; 2 natural (right), 2 Czochralski synthetic (center) and two Verneuil flame fusion synthetic colorless sapphires (far left) are tested under longwave (Left image) and short wave uv (right image)

Although flux, and hydrothermal process has been used to synthesis colorless sapphire it is not viable because of the manufacturing costs, flame fusion and crystal pulling are more typical. (Elen. S, Fritsch E. 1999).

Fluorescence in Blue Sapphire

We can try to define natural blue sapphire as being colored by different chromophores such as Ti (Titanium) and or Fe (Iron) , which is what most gemologists already understand. It is also understood that Fe diminishes fluorescence but There are several things that can lead to an inert reaction while exposed to long wave ultra violet fluorescence in natural blue sapphire. .(Emmett J, Hughes R.W. 2005)

Richard Hughes writes “Natural sapphires grow at much lower temperatures, so Ti4+ is much less likely to pair up with Al vacancies. These lower temperatures also allow easier pairing of Ti4+ with other ions (usually Fe2+ or Mg2+) that prevent fluorescence. Another damper is the presence of Fe3+, which also kills fluorescence. And finally, as the crystal sits in the ground for millions of years, diffusion slowly takes place, allowing the Ti4+ to slowly pair up with other ions, thus killing the fluorescence.”

Things do however get very interesting when we compare reactions of natural heated blue sapphire and synthetic blue sapphire under short-wave UV; Synthetic blue sapphire and heat treated blue sapphire with low iron are colored in similar ways. Coloring mechanism and growth condition cause synthetic blue sapphire to emit a chalky luminescence when exposed to short wave UV light. Heat treated blue sapphire with low amounts of “Fe” (for example blue sapphire from Sri Lanka) can also produce this sort of chalky fluorescence yet this will only be emitted in natural growth zones (Emmett J.; Hughes R.W. 2005) (Crowningshield 1970)

Direct Vision Spectroscope

A handheld spectroscope hardly has any diagnostic value in determining natural vs synthetic corundum. Nonetheless a spectroscope is a great tool to determine if the material we are dealing with is corundum or another type of gemstone. In blue sapphire, the infamous three-line complex in the violet end of the spectrum is diagnostic of natural origin. However, in my findings this does not always appear or can sometimes be distorted to look like one or two broad lines, therefore a direct vision spectroscope is otherwise not conclusive when trying to identify natural and synthetic corundum. (Gemmological Association of Great Britian 2009 D15 pg 7) (Hughes, R.W; Koivula J. 2005)

Figure 9 Photo courtesy of, John Harris; three lines at 450,460, and 471nm merge as a single band

Identification Based on Trace Elements using Energy Dispersive X-Ray Fluoresce (EDXRF) Spectroscopy

Accurate identifications can be made by a trained gemmologist using classical instruments and microscopic inspection. However, in cases when testing certain synthetics that are generally free from inclusions or contain ambiguous internal features advanced testing is needed. (Elen. S; Fritsch E. 1999)

The relationship between trace elements and chemical composition can be distinctive in determining if a gemstone is natural or synthetic. A laboratory can use several analytical techniques for determining trace element analyses; one of the most popular instruments in gemological laboratories is EDXRF, for it is portable and affordable. Experience with advanced testing equipment is invaluable one must be able to properly read the graphs and understand the data. (Herzog, F. 2015)

There is no one trace element that can determine if corundum is natural or synthetic across all varieties and synthetic processes therefore, it is vital to look at the full chemistry along with microscopic inspection to make a proper identification. Throughout my research the lack of Gallium (Ga) is associated with synthetic corundum, yet some flux melt synthetic Rubies like Dorious and Ramaura can actually contain slightly higher amounts of Ga. These synthetics can be identified from their trace amounts of Fe or even the presence of Pb. (The assumption is microscopic inspection has already been conducted on specimens to rule out lead glass filled ruby). (Herzog, F. 2015) (Elen. S; Fritsch E. 1999) (Muhlmeister S., Fritsch E., Shigley J.E., Devouard B., Laurs B.M. 1998) (Hanni, H.A. and Stern, B. 1982)

*Figure 10 is a graph of a 9.10 ct verneuil synthetic ruby and a 1.86ct Ramaura Synthetic Ruby from (Herzog, F. 2005), where Ga is below the detectable limits. The flux in this Ramaura synthetic ruby contain lead. In this example the Pb covers the Ga lines; From my findings if there is any presence of Pb one should be immediately skeptical on the gemstones authenticity.

Figure 10

*In the two samples tested by Franz Hezog I would like to note that both stones have low Fe and contains “Cr” as its only coloring agent.

This table taken from (Muhlmeister S, Fritsch E, Shigley J, Devouard B, and Laurs B 1998) shows the chemistry involved in the various synthetic methods. The different manufactures are generally irrelevant in the industry, most people just want to know if their gemstone is natural, or synthetic. Synthetic gemstones are created in more controlled environments compared to natural stones; when we look at melt growth synthetics, there are fewer additives therefore do not have as many additional elements. (Muhlmeister S., Fritsch E., Shigley J.E., Devouard B., Laurs B.M. 1998)

Figure 10.1

In this same article in Gems and Gemology 162 synthetic rubies and 121 natural rubies were tested. A summary of the ruby chemistry is listed below in figure 10.2 and10.3

Figure 10.2
Figure 10.3

We can see that in these samples the Dourous and even some of the Catham samples can have amounts of “Ga” that could suggest natural origin (If Ga is looked at alone); but when we compare the amounts of Vanadium and Iron in these stones with those of natural origin a proper identification can be made. So much can be said about the chemistry in corundum, which goes beyond the scope of this paper; I wanted to give a little bit of information about chemistry so that the reader can get a bigger picture on how complex identification can be.

In an article by Hanni, H.A. and Stern, B. 24 natural and 8 synthetic corundum’s are tested, and the amount of Ga were measured. To further support my statements on lower amounts of Ga suggesting synthetic origin I have posted the results from the experiment below.

Gallium (> about 200ppm) Gallium (< about 200ppm)

– Yellow ,blue, and red corundum from Sri Lanka

– Yellow and blue sapphire from Australia

– Sapphire and ruby from Burma

– Blue and pink sapphire from Montana

– Ruby from Umba, Tanzania

– Ruby from Kenya

– Sapphire from Thai land

– Star ruby from India

– Synthetic corundum, verneuil, red, blue, yellow, and padparacha

– Synthetic Ruby Chatham

– Synthetic Ruby Kashan

– Synthetic ruby Knischka

– Synthetic star ruby (Linde)

As a bonus I would also like to post chemical analyses of 3 synthetic and 7 natural samples originally from (Schneider,1977), this data was republished by (Henn U., Schrader H. 1985)

Although it is not uncommon for synthetics these days to contain higher amounts of Ga, it still can be very helpful (and diagnostic) to compare the relationship between Fe and Ga. (Hanni, H.A. and Stern, B 1982)

Figure 10.5


It can be very difficult to make the right identification when dealing with corundum. Although with enough experience certain features can be identified as indicative of natural and synthetic origin, microscopic inspection alone may not be enough to correctly make an identification. Fluorescence is an underutilized tool that can help identify colorless and sometimes blue sapphire. Advanced equipment such as EDXRF, can be the most diagnostic tool to confidently make an identification by comparing trace element chemistry; this should always be done in conjunction with microscopic evaluation.


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Perett, Karl Schmetzer and Adolf. Spring 1999. “Some diagnostic features of russian hydrothermal synthetic rubies and sapphires.” Gems and Gemology Volume 35 No.