“Lonsdaleite Diamond”: What Is All the Fuss About?

Maya Musa1*, Lisa Greggio2

1 Research Department director, Gulf Institute of Gemology, Sultanate of Oman. *Corresponding author: maya.musa@gulfgemology.com
2 Laboratory Director, Gulf Institute of Gemology, Sultanate of Oman

Lonsdaleite Diamond

Lonsdaleite is one of the most recent and rare materials researched by the scientists and it is characterized by amazing proprieties. One of its main characteristics is to be correlated with diamond, as far as the chemical composition and crystal structure are concerned, and with meteorites, because of its best-known formation process. Thanks to those proprieties, recently a material attractively called “Lonsdaleite Diamond” or “Stars Diamond” started to circulate in some gemstones and precious minerals markets.
The aim of the present article is to pull some threads and unveil Lonsdaleite, what it is, which are the researches and what is really possible to find in the trade.

The Lonsdaleite phase

Lonsdaleite, named in honor of Dame Kathleen Lonsdale [Fig. 1][1], originally describes the hexagonal carbon form [2] and represents one of the polymorphs of the carbon series, including the diamond and the graphite mineral phases [Fig. 2]. Indeed, Lonsdaleite’s crystal structure is hexagonal and the chemical composition is C; for these reasons it is also called “hexagonal diamond”.
This mineral has been discovered for the first time about 50 years ago in the Diablo Canyon iron meteorite; its formation was attributed to shock-induced transformation of graphite within the meteorite upon impact with the Earth [3]

Kathleen Lonsdale Lonsdaleite
Fig. 1 Kathleen Lonsdale with crystal models, photographer unknown, c. 1946. Courtesy of Prof. Ian Wood, UCL Earth Sciences, for “Disrupters and Innovators: Journeys in gender equality at UCL”, Exhibition 3 September 2018 - 3 February 2019, Octagon Gallery [1]. Kathleen Lonsdale was the crystallographer who established the structure of benzene by X-ray diffraction methods in 1929. She also worked on the synthesis of diamonds, and was a pioneer in the use of X-rays to study crystals
Diamond graphite lonsdaleite Crystal Structure
Fig. 2 Crystal structures of diamond, graphite and lonsdaleite. Image by Popov, Gorne, Tchougreeff and Dronskowski, 2019 [12]
Such impact diamonds are polycrystalline aggregates composed predominantly of cubic and hexagonal diamond nanoparticles and residual crystalline graphite [4]. However, this definition is nowadays strongly discussed by scientists: Németh et al. (2014) hypothesized “lonsdaleite” does not exist as a discrete material and demonstrated it to be a faulted and twinned cubic diamond. He has also shown that other reported carbon polymorphs can be explained by twinning and stacking faults [5].On the other hand, both Kraus et al. (2016) and Turneaure et al. (2017) have proved lonsdaleite formation (alongside with diamond), as a separate species, generated by shock compression of graphite[6, 7]. Lonsdaleite has also received much attention because of its potentially superior mechanical properties, such as compressive strength, hardness and rigidity, thought to rival or exceed those of cubic diamond[8, 9]. However, these exceptional properties have not been proven experimentally because of the inability to synthesize lonsdaleite as a pure phase[5]. For example, during the synthesis process of the nano-polycrystalline diamonds (NPD), the genesis of Lonsdaleite lamellaes, strongly associated with the cubic diamond grains, have been hypothesized[10].
Canyon Diablo meteorite with a diamond region
Fig. 3 Left, Polished section of shocked Canyon Diablo meteorite with a diamond-bearing region indicated by the arrow (Specimen ASU34.364), scale cube is 1 cm. Right, Individual Canyon Diablo “Diamonds”. Distance between scale markers is 1 mm. Images by Laurence A.J. Garvie & Peter Nemeth, 2009 [11]

It must be underlined that lonsdaleite is a very rare material on Earth, found as small grains associated with cubic diamonds and graphite in iron meteorites [Fig. 3]; the layers in the same grain corresponding to diamond, graphite and lonsdaleite can be distinguished only by very sophisticated instruments, able to work in nanometric scale. Moreover, nowadays a massive and relatively big (millimetric scale) pure crystal of lonsdaleite, neither natural or synthetic, is not reported.

The Case

In June 2020 GIG Laboratory received a near round polished, white and opaque sphere, weighting about 100 ct, for analysis [Fig.4]. Maintaining the ethic of the laboratory, the team didn’t ask anything about the sample history before the end of the tests, performed in order to identify the material. The lab team carried out several analyses on the sample, from the standard observations [Table 1] to the most high-tech applications. Particularly, through the optical microscope, a conglomerate of very small grains structure has been observed. Moreover, the bead presented uncommon fluorescence at the wood lamp test: patchy yellow with medium intensity at long wave (365 nm) and medium chalky blue at short wave (254 nm). Finally, the specific gravity resulted 3.53. The chemical composition has been carried out by the ED-XRF and the elements’ relative distribution is reported in table 2. The high concentration of Aluminum must be noted. The chemical information has been integrated by micro-Raman analysis. This technique reveals itself particularly useful to identify mineral phases, without any sample preparation. Thanks to the optical microscope coupled with the Raman spectrophotometer, the instrument source was focused on different points of the sample’s surface and a spectrum like the one reported in Figure 5 has been carried out. The bands at 418 and 380 cm-1 observed in the Raman spectrum are consistent with those reported for the Corundum mineral phase [13]. Crossing all the data acquired, the final diagnosis for the case-sample resulted consistent with alumina ceramic compound.
Lonsdaleite Diamond
Fig.4 The sample analyzed by GIG Laboratory in June 2020 and sell as Lonsdaleite Diamond.

“Alumina Ball” or “Lonsdaleite Diamond”?

For a gemological laboratory, one of the saddest moments is when a fake material must be disclosed to the customer, especially if the laboratory is consulted for the report service after the transactions havebeen completed and the object has been purchased. The case-sample presented above was bought as “Lonsdaleite diamond”. After this discovery, the GIG team decided to examine more in depth about the presence of Lonsdaleite in the jewellery trade of the Gulf area. As far as we know, the number of lonsdaleite samples, or presumed such, is not too high, but they have all the same aspect: round and near round white sphere. In order to furtherly clarify our thoughts, we contacted other laboratories to find out if someone else analyzed the same material and we received confirmation that a similar type of material has been analyzed recently by Gubelin Gem lab, with the identical conclusions of GIG: ceramic alumina compound [14]. What is particularly interesting is the online diffusion of videos describing these spheres as “lonsdaleite diamonds”, “stars diamonds” or “hexagonal diamonds”, their fascinating characteristics, and in some cases, how to distinguish between “lonsdaleite balls” and “alumina balls”, using do-it-yourself systems. Indeed, if the lonsdaleite is extremely rare phase, “alumina balls” are very common and relatively cheap material, synthesized for industrial purposes. Therefore, the consequent question is: can the Lonsdaleite appear as a white sphere? As far as we know, none peer review scientific references report a lonsdaleite description consistent with a white sphere!
Lonsdaleite Raman
Fig. 5 Raman spectrum acquired by Renishaw InVia spectrophotometer, focusing the instrument laser source (514.5 nm) on the surface of the sample, in the range between 150 and 1800 cm-1. The bands at 418 and 380 cm-1 observed in the Raman spectrum are consistent with those reported for the Corundum mineral phase [13].

Tables

Table 1 Gemological characteristics of the sample analyzed in June by GIG Laboratory
Table 2 Chemical composition carried out by ED-XRF technique, ThermoFisher Arl Quant’X, using UniQuantTM standardless method. It must be noted the high concentration of alluminium.

References
[1] https://www.ucl.ac.uk/culture/projects/disrupters-and-innovators.
[2] Bundy F P, Kasper J S (1967):Hexagonal diamond-a new form of carbon, Journal of Chemical Physics, 46, 3437-3446.
[3] Goryainova S. V., Likhachevaa A. Yu, Ovsyuka N. N. (2018): Raman Scattering in Lonsdaleite, Journal of Experimental and Theoretical Physics, 127(1), 20-24.
[4] Ohfuji H., Irifune T., Litasov K. D., Yamashita T, Isobe F, Afanasiev V. P., Pokhilenko N. P. (2015): Natural occurrence of pure nano-polycrystalline diamond from impact crater, Scientific Report, 5, 14702.
[5] Németh P., Garvie L. A.J., Aoki T., Dubrovinskaia N., Dubrovinsky L., Buseck P. R. (2014): Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material, Nature Communications, 5, 5447.
[6] Kraus D., Ravasio A., Gauthier M., Gericke D.O., Vorberger J., Frydrych S., Helfrich J., Fletcher L.B., Schaumann G., Nagler B., Barbrel B., Bachmann B., Gamboa E.J., Goede S., Granados E., Gregori G., Lee H.J., Neumayer P., Schumaker W., Doeppner T., Falcone R.W., Glenzer S.H., Roth M. (2016): Nanosecond formation of diamond and lonsdaleite by shock compression of graphite, Nature Communications, 7, 10970.
[7] Turneaure S. J., Sharma S. M., Surinder M., Volz T.J., Winey J. M., Gupta Y. M. (2017): Transformation of shock-compressed graphite to hexagonal diamond in nanoseconds. Science Advances, 3(10), 3561.
[8] Pan Z., Sun H., Zhang Y., Changfeng C. (2009): Harder than Diamond: Superior Indentation Strength of Wurtzite BN and Lonsdaleite, Physical Review Letters, 102 (5), 055503.
[9] Quingkun L., Yi S., Zhiyuan L., Yu Z. (2011): Lonsdaleite – a material stronger and stiffer than diamond, Scripta Materialia, 65, 229–232.
[10] Skalwold E. A. (2012): Nano-Polycrystalline Diamond Sphere: a gemologist’ s perspective, Gems & Gemolgy, Summer, 128 – 131.
[11] Garvie L. A.J., Nemeth P. (2009): The structure of Canyon Diablo “Diamonds”, 40th Lunar & Planetary Conference.
[12] Popov I.V., Gorne A.L., Tchougreeff A.L., Dronskowski R. (2019): Relative stability of diamond and graphite as seen through bonds and hybridizations, Physical Chemistry Chemical Physics., 21, 10961-10969.
[13] Schubnel H. J., Pinet M., Smith D. C., Lasnier B. (1992): La microsonde Raman en gemmologie, E. Ruskone` Ed. (A.F.G., Paris) p. 25.
[14] Gubelin Gem Lab, Private Communication.

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