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Current position

I am a Post Doctoral Fellow at the National Museum of Natural History (NMNH), Smithsonian Institution, Washington D.C. I am currently doing research on the cause of color in pink diamonds, using different methods of spectroscopy and chemical analysis. We know that in most diamonds, color is zoned in some defect planes that are called graining, but the exact nature of the color is not known.

Another purpose for my coming at the NMNH is to study diamond in jewels of the National Gem Collection, as for example the Napoléon Diamond Necklace, jewelry of almost 200 years old.

Fig. 1: Pink diamond from Brazil, NMHN collection.

Previous subjects of interest

My two main subjects of research concern both diamonds and gem opals.

DIAMONDS. My first work on diamonds concerns some absorptions which are enhanced or decrease by light exposure, that we call photoinduced absorptions. We have found in three yellow to orange natural, treated-color diamonds that the well-known H1b and H1c centers may also be photoinduced. These defects are typical of treatment by irradiation followed by annealing. Furthermore, as the H1b/H1c absorptions either appear or their intensity increases (by a factor of three under ultraviolet exposure), that of the 595 nm decreases. Photoinduced absorptions disappear completely after a few seconds exposure to visible light and at the same time the 595 nm absorption increases back to its original height. However most diamonds with H1b and H1c centers do not exhibit this behavior. It seems that at least one condition for this photoinduced effect is the simultaneous presence of the H1b, H1c, H3, GR1 and 595 nm centers. The centers implied are all related to nitrogen (and to the existence of a treatment), the formation of H1b/H1c centers coming from the trapping of the 595 center at A- and B-aggregates respectively. A possible explanation is tunneling of an electron from the 595 center to the H1b/H1c centers. One practical gemological application of this is the detection of low concentration of H1b and/or H1c defects in colored diamonds. Indeed, in some cases an exposure to UV light during IR spectra acquisition could enhance the H1b/H1c centers, proving the existence of treatment by irradiation followed by annealing.

OPALS. I obtained my PhD in Material Sciences in France studying nanostructure, physical properties and the mode of formation of opals. Here are some examples of the results obtained.

Fig.2: Play-of-color fire opal-CT from Mexico.

Nanostructure. We have demonstrated that the well-known structure of opals as being made of a perfect network of spheres of 150 to 300 nm in size is in fact not the most widespread. First, this concerns only opal-A, and specifically the one showing this valuable play-of-color (Fig. 2). However, spheres are not so well individualized, and have very often cement between them. The other opal-A which do not show this play-of-color but are valuable for their body color ("common opals") are made of spheres too, but they are not well-organized, because 1- they do not have a homogeneous size (fig. 3) 2- they are imperfectly shaped, 3- they are too large or too small to diffract light, or 4- they are just not well-ordered. The elementary building block of these spheres is what we called "nanograins", i.e. grains of about 25 nm in diameter, and are arranged in a concentric way or less often in a radial way.

Fig. 3: Micrograph of a common opal-A from Slovakia. Spheres have different size, and are made of nanograins arranged in a concentric way.

The other variety of opal is the one called opal-CT, which are not amorphous as opal-A but "microcrystalline". They are also made of nanograins but they never arrange in real spheres. Common opal-CT could show different structure, up to four degrees of order. Indeed, the nanograins could be just individualized, without any order making a somewhat granular structure. They could arrange in fibers, then tablets or platelets, or in the so-called lepispheres (spherical aggregates of plate-like crystallites of cristobalite) which represent the third degree of order. The fourth degree of order is obtain when the lepispheres arrange themselves in a perfect network, and when they have the right dimension, opal-CT displays play-of-color. Very often, lepispheres are cemented in gem opals, and the structure is not directly observable (on the contrary to biogenic opal) in fresh breaks: HF attack is required to have access at this information.

Fig. 4: Micrograph of a common white opal-CT from Mexico. Nanograins are arranged in platelets.

Mode of formation. We have done some chemical analysis (by dilution by ICPMS) on opal-A and-CT from 11 countries, in order to establish which impurities are presents and in which concentration. The main impurities present are, in order of decreasing concentration, Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts are Ba, followed by Zr, Sr, Rb, U, and Pb. The geochemistry of an opal is shown to be dependant mostly on the host rock, even if modified by weathering processes, as the REE patterns are very similar except for a factor of dissolution. We determined that it is possible to separate opals depending on their geologic origin: Ba content, as well as the REE pattern normalized to chondrite, are key to separate sedimentary opals (Ba >110 ppm, Eu and Ce anomalies) from volcanic opals (Ba < 110 ppm, no Eu or Ce anomaly). We can also separate opals depending on their geographic origin using several impurities. The Ca content (and to a lesser extent that in Mg, Al, K and Nb) helps to distinguish gem opals from different volcanic environments. In terms of origin of color, for example, greater concentrations of iron induce darker colors (from yellow to "chocolate brown"). This element inhibits luminescence for concentrations above 1000 ppm, whereas already a low content in U (≤1 ppm) induces a green luminescence.

Education

• 2003-2006: PhD in Materials Physics, Institut des Matériaux Jean Rouxel (IMN) Nantes, France. Advisors Prof. Fritsch and Dr. Cornen. Subject: Relations between nanostructure, physical properties and mode of formation of opal-A and -CT.
• 2003-2005: Advanced Gemology Diploma (Diplôme d’Université de Gemmologie, DUG), University of Nantes, France. Subject: New photoinduced absorptions in diamond: H1b, H1c and the 4850 cm^–1 system.
• 2002-2003: Masters degree (DEA) "Magmatic and metamorphic process, Volcanology" University of Clermont-Ferrand, France. Six-month research project. Advisors: Prof. Vielzeuf, Devouard and Boivin. Subject: Mineralogical study of sapphires from Sioulot, Mont Coupet and Menoyre; Determination of their geological origin.
• 1998-2002: Bachelor of Earth and Planetary Sciences (Maîtrise), University of Nantes. Scientific work leading to a publication (see below). Advisors Prof. Fritsch and Lasnier. Subject: Fibrous pink opals.

Teaching

• 2006-2007: Temporary researcher and teacher assistant at the University of Nantes, France in Earth Sciences. Teaching (88h) Geosciences for first year students and Gemology (classical gemology course) for second year student.
• 2004-2006: Teaching classical gemology course, University of Nantes, France. 44 hours, for second year geology students.

Skills

• Equipments used during research work: Autonomous for scanning electron microscope and associated microanalysis and cathodoluminescence spectrometry; UV-visible-near infrared spectrometry; infrared spectrometry; Raman scattering spectrometry; luminescence (cathodoluminescence, fluorescence, phosphorescence) spectrometry. User of X-ray diffraction, gas mass spectrometry, laser ablation inductively coupled plasma mass spectrometry, ToF SIMS.
• Software used: Word, Excel, Photoshop, Origin, Power Point, Internet.
• Languages: English, good level.

Publications

Gaillou E., Devouard B., Vielzeuf D., Boivin P., Rochault J., Valley J., Harris C. (2008) Les saphirs du Mont Coupet, de Menet et du Sioulot : trois gisements du Massif Central à caractéristiques pétrogénétiques contrastées. In Press, Le Règne Minéral.

Gaillou E., Delaunay A., Rondeau B., Bouhnik-Le Coz M., Fritsch E., Cornen G., Monnier C. (2008) The geochemistry of opals as evidence of their origin. In Press Ore and Geology Reviews. [download pdf]

Gaillou E., Post J.E. (2008) Smithsonian collection provides treasure store of gemstones. InColor, Spring 2008, pp. 31-34

Gaillou E., Post J.E. (2007) An examination of the Napoléon Diamond Necklace. Gems and Gemology, vol. 43, n° 4, pp. 352-357. [download pdf]

Karampelas S., Gaillou E., Fritsch E., Douman M. (2007) Les grenats andradites–démantoïdes d’Iran : zonage de couleur et inclusions. Revue de gemmologie a.f.g., vol. 160, pp. 14-20.

Karampelas S., Gaillou E., Fritsch E., Douman M. (2007) Color-zoned andradite-demantoid from Iran, with calcite inclusions. Gems and Gemology, vol. 43, n° 1, pp. 65-67.

Fritsch E., Gaillou E., Rondeau B., Barreau A., Albertini D., Ostroumov M. (2006) The nanostructure of fire opal. Journal of Non Crystalline Solids, vol. 352, pp. 3957-3960. [download pdf]

Gaillou E., Delaunay A., Fritsch E., Bouhnik-le-Coz M. (2006) Geologic origin of opals deduced from geochemistry. Gems & Gemology, vol. 42, n°3, p. 107.

Gaillou E. (2006) L’opale : un nanomatériau naturel. Mensuel de l’Université, n°11, section « Études » (online only at http://www.lemensuel.net/L-opale-un-nanomateriau-naturel.html).

Gaillou E. (2006) Relations entre nanostructure, propriétés physiques et mode de formation des opales A et CT (Relationship between nanostructure, physical properties and formation mode of opal-A and -CT). Université de Nantes, France, PhD Thesis, 307pp. [download pdf].

Gaillou E., Rondeau B., Fritsch E., Bouhnik-Le-Coz M., Cornen G., Ostroumov M. (2005) Toward a geochemistry of opals. Geochimica et Cosmochimica Acta, vol. 69, n° 10, p. A279. [download pdf]

Gaillou E., Mocquet B., Fritsch E. (2005) A new material from Madagascar: A mixture of cristobalite and opal. Gems & Gemology, vol. 40, n° 4, pp. 339-340.

Gaillou E. (2005) Nouvelles absorptions photoinduites dans le diamant : H1b, H1c et système à 4850 cm^–1. Revue de gemmologie a.f.g., n° 155, p. 23.

Fritsch E., Gaillou E., Ostroumov M., Rondeau B., Devouard B., Barreau A. (2004) Relationship between nanostructure and optical absorption in fibrous pink opals from Mexico and Peru. European Journal of Mineralogy, vol. 16, pp. 743-752. [download pdf]

Forthcoming publications

Gaillou E., Fritsch E., Aguilar–Reyes B., Rondeau B., Barreau A., Ostroumov M. Common gem opal: An investigation of micro- to nano-structure. Submitted to American Mineralogist in November 2006.

Gaillou E., Fritsch E., Notari F. Photoinduced absorptions of the H1b and H1c centers in some natural treated diamonds. Accepted at Diamond and Related Materials in November 2007.

Conferences

Gaillou E. (2008) Color in gem opals. Conference at the Mineralogical Society of Washington DC, April 2nd. Invited speaker.

Gaillou E. (2008) Color in gem opals. 1417th Meeting of the Geological Society of Washington, January 23rd, Whashington, DC, USA. Invited speaker.

Gaillou E., Fritsch E., Notari F. (2007) Photoinduced absorptions of the H1b and H1c centers in some natural treated diamonds. Diamond Conference, Warwick, UK, July 11th-13th 2007 (poster).

Fritsch E., Gaillou E., Rondeau B., Ostroumov M., Aguilar-Reyes B., Barreau A., Albertini D. (2007) Shining new lights on opals. Annual Conference of the Scottish Branch of Gemmological Association, May 4^th-7^th> 2007 (oral).

Fritsch E., Rondeau B., Gaillou E., Gauthier J.-P., Massi L. (2007) Questions actuelles sur l’apparence colorée des matériaux gemmes. GDR Couleur, March 14^th, CNRS, Paris, France (oral).

Gaillou E. (2006) Inclusions minérales dans les saphirs : aide à la détermination de l’origine géologique et géographique. Scientific days on vitreous inclusions, Nancy, France, November 30th and Decembre 1st 2006 (oral). Invited speaker.

Gaillou E., Delaunay A. Fritsch E., Bounik-Le Coz M. (2006) Geologic origin of opals deduced from geochemistry. Gemological Research Conference (GRC), August 26th-27th, San Diego, California, U.S.A (oral).

Gaillou E., Fritsch E., Notari F. (2005) New photoinduced absorptions in diamonds: H1b, H1c and 4850 cm^–1 system. Diamond 2005, September 11^th-16^th, Toulouse, France (poster).

Gaillou E., Rondeau B., Fritsch E., Bouhnik-Le-Coz M., Cornen G., Ostroumov M. (2005) Toward a geochemistry of opals. Goldschmidt Conference 2005, 20-25 May, Moscow, Idaho, USA (oral).

Gaillou E., Fritsch E., Rondeau B., Aguilar B.O., Faulques E., Ostroumov M. (2005) L’opale, un nanomatériau naturel. Colloque Matériaux Pays de la Loire. La Chapelle/Erdre, Juillet 8^th-9^th (poster).

Gaillou E. (2005) Les saphirs magmatiques du Massif Central. VI^es Rendez-Vous Gemmologiques de Paris, March 5th, Paris, France (oral). Invited speaker.

Fritsch E., Ostroumov M., Rondeau B., Aguilar-Reyes B.O., Barreau A., Gaillou E., Albertini D., Wery J. (2003) Nano- to micro-structure of natural gem opals: relation to deposition and growth conditions. Materials Research Society Fall 2003. Boston, MA, USA, 3 December (oral).