AGTA GTC's Laboratory Update for May 31, 2005

In this message…

Following the Silk Road: Rutile silk in corundum
GemmoBasel 2005
Testifyin’ – Simon Watt
The AGTA GTC in Las Vegas

Following the Silk Road: Rutile silk in corundum

by John I. Koivula and Richard W. Hughes

If I'd only seen through the silky veils of ardor…

– Joni Mitchell

What follows is a love story. We admit it. Our heart is on our sleeves. We are smitten, head-over-heels in love with the exotic and beguiling internal lattice that gemologists call “silk.”

How do we love thee? Let us count the ways. You are the one that lets some of our rubies and sapphires sparkle like the night sky, your asterism such a wonder of nature. Not to mention your beauty as you wink up at us when the light is just so. And finally, you are our guardian, our coalmine canary, a tiny thermometer in our gems, willing to sacrifice your own life to let us know when Man’s ego has grown so large that he thinks he can play God. For this alone, we sing your praises.

Figure 1. A secret silken world

Daggers of rutile lance their way through the interior of this untreated Mogok sapphire. The presence of such perfectly formed rutile needles are proof that this sapphire has not been heated to a high temperature. Photo: John I. Koivula/microWorld of Gems

Just what is this inclusion we call silk? It is actually composed of tiny crystals formed through a process called exsolution, the “unmixing” of a solid solution. At high temperatures, crystals have more defects and a more-expanded lattice, and thus are better able to absorb impurities. As the crystal cools, defects are reduced. This may force impurities to crystallize out. But because of the constraints placed on their movement by the solid host, impurity atoms are unable to travel large distances. Therefore, rather than forming large crystals, they migrate short distances to form multitudes of tiny needles, plates and particles, along the directions in the host where space permits.

Figure 2. Exaggerated and simplified atomic view of exsolution in corundum

During exsolution, solute atoms migrate together to form their own crystals within the host. The orientation of these crystals is governed by the host structure. As a result, they are exsolved in a specific pattern. Within corundum, rutile (TiO₂) unmixes in the basal plane, parallel to the faces of the second-order hexagonal prism {1120}.
Illustration © Richard W. Hughes/RWH Publishing & Books

One of the keys to recognizing exsolved inclusions is that they always form in a specific pattern within the host. That pattern may be different for different minerals crystallizing within the same host material (for example, rutile is exsolved in corundum in three directions crossing at 60/120° in the basal plane). Virtually all tiny, oriented needle, particle and platelike inclusions found in minerals are formed via exsolution. These inclusions give rise to asterism and cat’s eye phenomena. The best manifestation of this is the concentrations of rutile and hematite-ilmenite silk and needles in corundum.

Figure 3. Gossamer…

A rutile silk spider’s web spun beneath the facets of an untreated Sri Lankan sapphire. Photo © Richard W. Hughes/RWH Publishing & Books

Among the most diagnostic features of corundum is the white clouds of exsolved rutile (TiO₂). According to Edward Gübelin, Gustav von Tschermak was the first to identify rutile in corundum in 1878. Such clouds vary from dense concentrations, which follow, and distort, the crystals’ color zoning, to thinly-woven tapestries. At times, only slender threads or particles are visible, while in other cases knife or dart shapes appear (see Figure 1). Closer examination reveals many of these to be twin crystals with tiny v-shaped re-entrant angles visible at the broad end. They are flattened so thin in the basal plane that, when illuminated with a fiber-optic light guide from above, bursts of iridescent colors are seen, due to the interference of light from these microscopically-thin mineral lances.

The needle clouds just described are termed silk, in analogy to their threadlike pattern and are responsible for the asterism, or star effect. Not only rutile may form silk in corundum; hematite (Fe₂O₃), ilmenite (FeTiO₃) or hematite-ilmenite mixtures have been reported. Rutile in corundum tends to unmix parallel to the faces of the second-order hexagonal prism {1120}, intersecting in three directions at 60/120° in the basal plane. Hematite-ilmenite exsolves in the basal plane parallel to the first-order hexagonal prism {1010}. Thus when both rutile and hematite-ilmenite are present in the same crystal, a 12-rayed star is possible.

silk before and after heating

Figure 4. Effects of high-temperature heat treatment on rutile silk in corundum

Left: In corundum, rutile exsolves in three directions in the basal plane, parallel to the faces of the second-order hexagonal prism {1120}. Before heating, rutile silk consists of needles which, when highly magnified, are often tiny arrow- or dart-shaped twins with small reentrant angles at the wide end. The rutile needles are extremely thin in cross-section and are generally flattened in the basal plane. Overhead fiber-optic illumination with the microscope will reveal such details.
Right: High-temperature heat treatment causes rutile to be partially dissolved into the corundum, but traces often remain behind, visible with fiber-optic lighting. Each needle, rather than coalescing into one globule, instead dissolves into a series of tiny droplets arranged in the same pattern as before heating. This partially-dissolved silk may be indicative of heat treatment. However, minute exsolved particles, which can occur naturally, may be confused with the partially-dissolved silk of heated gems. The presence of long needles and dart- and arrow-shaped rutile is a strong indication that the gem has not undergone high-temperature heat treatment.
Illustration © Richard W. Hughes/RWH Publishing & Books

heated silk

Figure 5. Canary in a coal mine

When a ruby or sapphire is heat treated at a high temperature, rutile silk is partially resorbed into the crystal, thus clarifying it. But rutile is rarely completely pure. The more impurities present, the more visible the silk “skeletons” following treatment, as shown above. Thus rutile acts as an internal thermometer in the gem, letting us know when Man is trying to play God. Photo © Richard W. Hughes/RWH Publishing & Books

rutile silk

Figure 6. Arrows of love

Dart-shaped silken arrows of love. Can anyone not be smitten with such stunning displays? Untreated Sri Lankan ruby. Photo: John I. Koivula/microWorld of Gems

If I'd only seen through the silky veils of ardor…
What a killing crime this love can be…

– Joni Mitchell

Some shirk battle, afraid of missiles sent their way. Not us. Like Tibetans rounding Mount Kailash, we will gladly prove our devotion on our hands and knees. We are willing pilgrims along the Silk Road. We fear no slings; we relish these god-sent daggers. Let them penetrate our hearts, infect our souls.

What is love? We have gazed at and through the silky veils of ardor. And having journeyed to the other side, we wish for nothing less than to die a sweet slow silken death. We promised you a love story. Oh what a killing crime this love can be…

GemmoBasel 2005

Gemologists from around the world recently gathered in Basel, Switzerland for a three day gemological conference hosted by the SSEF Swiss Gemmological Institute in celebration of Director Prof. Henry A. Hänni’s 60^th birthday. Those in attendance representing the AGTA’s Gemological Testing Center, invited to give lectures were Laboratory Director, Dr. Lore Kiefert, also one of the primary organizers of the SSEF GemmoBasel conference, and AGTA-GTC Chief Gemologist, John Koivula.Dr. Kiefert’s presentation, titled “Origin reports: Advantages, disadvantages, limits, pitfalls” addressed country-of-origin determination and the inherent problems faced by laboratory gemologists in the preparation of such documents. Mr. Koivula gave a speech on “The Microworld of Gems,” which highlighted recent discoveries of new and unusual inclusions and other micro-characteristics of a wide variety of natural gems as well as treated and synthetic materials. Having personally visited Burma (Myanmar) more than a dozen times to explore the gem markets and mines Richard W. Hughes was also invited to give a lecture entitled “Mogok: The Valley of Rubies,” but was unable to personally attend. His lecture was presented instead by Edward Boehm, who also chaired the “Gemstone” session. GemmoBasel 2005 was well attended, enjoyable and gemologically educational, so much so that gemologists may see future conferences organized by the SSEF under the GemmoBasel banner.

Lore Kiefert, Henry Hanni and John Koivula

Bosom buddies

AGTA Director Lore Kiefert, along with Henry Hänni of the SSEF and the AGTA’s John Koivula at GemmoBasel 2005.

Lore Kiefert

Waxing poetic

AGTA GTC’s Director, Lore Kiefert, dissecting the conflicting interests of gemstone origin reports at GemmoBasel 2005.

Testifyin’
Thank you so much for your “man in the street” or “gem dealer in the wild” setup for ultraviolet fluorescence testing of heated corundum. While in no way does this replace my need for the AGTA GTC lab reports, it is another hammer in my identification toolbox. It is truly refreshing to have someone in the scientific world relate to us neophytes in terms of gemology and dollars. Bravo!
Making my job easier and making me more professional is wonderful added value to my AGTA membership and I look forward to my "continuing education."

Simon Watt, Mayer & Watt

The AGTA GTC in Las Vegas

Once again, the AGTA GTC will be participating in The JCK Show - Las Vegas 2005, offering a range of gemological services, such as:

Identification reports for all kinds of gems, including the identification of clarity enhancement fillers
Country-of-Origin reports for ruby, sapphire and emerald.

The AGTA Pavilion has special dates and times! The AGTA Pavilion opens and closes one day before the main JCK show. The AGTA GTC Mobile Laboratory, located in the AGTA GemFair Cultured Pearl & Jewelry Pavilion, is open Thursday, June 2nd–Monday, June 6th. Hours are 10:00 AM to 6:00 PM on June 2nd and 9:00 AM to 6:00 PM from June 3rd–6th. The AGTA GTC Mobile Laboratory is planning to deliver the reports to clients within 1–2 days and the services are going to be available during the show time.