Welcome to AGTA GTC's Laboratory Update for October 4, 2005

In this message

  1. AGTA GTC Staff Lecture to Raman Conference
  2. Dangerous Curves: Verneuil Synthetics Reexamined

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AGTA GTC's Lore Kiefert Lectures at Raman Conference
The 3rd conference on the application of Raman Spectroscopy in Art and Archaeology was held from August 31st to September 3rd in Paris. This conference primarily deals with pigments and dyes used for paintings, pottery, and archaeological artifacts.

Lore Kiefert lecturing

Figure 1. Dr. Lore Kiefert lecturing to the Raman conference in Paris.

Raman conferences in all subjects are getting more popular at the same rate that Raman spectrometers are getting more affordable (Figure 2). 

Raman Spectrometer

Figure 2. Renishaw Raman spectrometer
The Raman spectrometer is extremely useful in the non-destructive identification of inclusions in gemstones, as well as fingerprinting rare gem materials. It also can be used to identify certain types of fillers used in treated emeralds, along with HPHT treated diamonds. This is just one of the advanced gem-testing instruments in residence at the AGTA GTC.

These spectrometers found their way into gemological laboratories approximately ten years ago, but gemologists are rarely found at these conferences. However, the conferences are a good meeting point for scientists working in related fields, and many gemological problems may have already been solved by these other scientists. In this conference, contacts could be established to the directors of laboratories of the Getty Museum in California and the Metropolitan Museum in New York, both of which have similar and complimentary instruments to what the AGTA GTC has.
     Our laboratory director, Dr. Lore Kiefert, has been visiting Raman conferences since 1996, giving poster presentations and lectures about the use of Raman spectroscopy in gemology. The ArtRaman 2005 was a good forum for her lecture on corals, because these organic gemstones receive their color by organic compounds such as carotene, but often you can find dyed corals or coral imitations on the market. Gemologists do not generally identify organic dyes, but differences between natural colored corals and dyed corals are easily picked up with Raman spectroscopy.
     Her talk on corals was well received, and various experts from France, Belgium and the United Kingdom offered their help for identifying the dyes used not only in corals, but also in pearls and gemstones such as quartz and jadeite. This will hopefully lead to cooperation in shedding light on treatment processes used in the past and at present.

Figure 3. Dyed coral
Corals are not only dyed red, but also pink and orange to imitate their natural counterparts. The three pieces to the left are dyed corals with their typical striations. The beads on the right are shell beads, which were dyed and then coated with a lacquer. Photo: Lore Kiefert.

Figure 4. Raman spectrum of natural colored and dyed coral
The Raman spectrum of natural colored red coral (bottom spectrum) shows two distinct bands at 1128 cm-1 and 1517 cm-1, which are attributed to carotene, the coloring agent not only for natural colored red and pink corals, but also for natural colored pink pearls. On the contrary, artificially colored corals show several bands between 800 and 1800 cm-1. These are due to artificial pigments. Spectra taken at SSEF Swiss Gemmological Institute in 1997.

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Dangerous Curves:
A Reexamination of Verneuil Synthetic Corundum

While virtually all gemologists are aware of the curved growth lines and gas bubbles in Verneuil (flame-fusion) synthetic corundum, few are aware of exactly why they occur and how they are distributed in these stones. We hope to clarify that with this article.

Figure 1. A selection of half boules grown by the Verneuil process, illustrating many of the colors these crystals are grown in. Due to stress in the growth process, a parting plane forms in all Verneuil grown synthetic corundums. Splitting boules along this plane prior to cutting relieves this stress. Photo © Richard W. Hughes/RWH Publishing & Books.

Curved Growth Lines
Curved growth lines are a prominent feature of virtually all Verneuil synthetic corundums. While many believe they are similar to the growth rings of a tree, the truth is quite different. Trees and natural crystals grow outward in all directions from a central point. In trees, these lines are concentric, while in natural crystals, they are angular, but both form from a central point.
     Verneuil synthetic corundum is quite different, growing in a single direction only (up) via feed material dropped from above. Since the growth begins from a tiny molten droplet, the growth lines form in semicircular layers following the top surface of that droplet (see Figure 2). Thus they are not concentric.
     These lines are seen by looking at a slightly oblique angle towards the top surface of the boule, as shown in Figure 2. Unlike a tree, if one looks parallel to the length of the boule, no lines will be seen.

Figure 2. Distribution of curved growth lines and gas bubbles in Verneuil synthetic corundum. Curved growth lines are best viewed by looking at an angle slightly oblique to the boule's length. Gas bubbles usually occur in layers that follow the curved growth lines. When the bubbles are elongated, the elongation is usually at right angles to the direction of the curved growth lines, with the head of the bubble facing the top of the boule. This is because bubbles are formed by boiling of the molten top surface of the boule as it grows. Illustration © Richard W. Hughes/RWH Publishing & Books.

Curved growth lines in Verneuil synthetics come in two flavors. Sharp narrow lines termed "striae" are found in synthetic ruby and the vanadium-colored color-change synthetic sapphire. In all other varieties, the curved lines occur in dull-edged bands (best seen by placing a white diffusing filter over the well of the microscope). While one would not expect to find any lines in the synthetic colorless variety, examining the gem's short-wave UV fluorescence will often reveal the curved structure. Make sure to use proper eye protection for this.
     Why do Verneuil synthetic corundums display these lines? Mainly because the boules are grown in an uncontrolled atmosphere with a huge drop in temperature (thermal gradient) between the growing crystal and surrounding atmosphere. This produces stress and a lack of diffusion of elements in the growing crystal. These tiny structural and compositional differences translate into the curved lines we see.

Figure 3. Curved banding as seen in a half boule of Verneuil synthetic corundum. Note that the curvature is greatest at the outside edges. Photo © Richard W. Hughes/RWH Publishing & Books.

Gas Bubbles
Gas bubbles in the Verneuil product arise from excess oxygen in the torch, creating a flame temperature that is too hot. This causes a localized boiling on the surface, resulting in the capture of gas bubbles (see Figure 2).
When elongated, the bubbles usually run against the grain of the growth zoning. The heads of elongated bubbles generally point towards the surface of the boule (see Figure 4).
     Why is this important? Because elongated bubbles in a gem can tell you which directions to concentrate on for locating curved growth lines.

Figure 4. Elongated gas bubbles torpedo through curved striae within the depths of a Verneuil synthetic ruby. Photo © John I. Koivula/microWorld of Gems.

Verneuil synthetic corundum is by far the most common in the market. Thus it is vital that gemologists are familiar with the key identifying features - curved growth lines and gas bubbles.

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The AGTA Gemological Testing Center provides the industry and the public with a complete range of lab services, including gemstone identification, origin determination and pearl identification. Located in New York City, the laboratory is equipped with the latest, technologically advanced, investigative equipment.The AGTA GTC is committed to providing excellent service, superior value and outstanding quality. A complete list of services and detailed pricing information is available on our website, www.agta-gtc.org. Please contact us with any questions.

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