Libyan Desert Glass, A Peculiar Gem

Lisa Greggio1, Muna Al Bulushi2, Maya Musa3

1 GIG Laboratory Director
2 GIG Senior Gemology Analyst
3 GIG Research Department Director

Fig.1 The LDG analyzed, weighting 52.89 ct

Last October a client submitted a pretty and rather big transparent faceted gem, fashioned in a mixed style and triangular shape, and exhibiting a light greenish Yellow colour. It weighted 52.89 ct and its measurements were 30.67 x 29.70 x 15.52 mm [Fig.1]. He told to the Laboratory he thought it was a Heliodor (the Yellow to greenish Yellow variety of the Beryl species), but he just wanted to doublecheck. Based solely on the appearance, it could really resemble a Heliodor. It was only after performing the gemological analyses that we had the proof that it was not a gem of the Beryl species, but a quite rare gem, even much more interesting from a gemological point of view than a Heliodor.
Both standard and advanced techniques were applied. The sample presented RI = 1.450 and SG = 2.21; at the microscope we observed long gas bubbles and whitish flat inclusions. The reaction to the UV was None at UVL (365 nm) and Faint Yellow at UVS (254 nm).

Table 1 Chemical composition of the LDG sample, acquired by ED-XRF. The instrument used is ArlQuant’X by ThermoFisher scientific. The acquisition as well as the normalization and quantification calculations have been carried out applying the UniQuantTMStandardless method.

The chemical analyses performed by ED-XRF highlighted a composition mainly of Si and Al, with traces of Ti, Fe, K and Zr [Tab. 1].
Parallelly, the Raman spectra [Fig.2] revealed broad bands features.
According to Frost [et al., 2010] two broad bands – one around 480 cm-1 and the other at 820 cm-1 – are typical for glassy silicate materials. It must be noted that the common industrial Silicon glass presents a completely different Raman spectrum, with an asymmetrical broad band at about 550 cm-1.

Fig. 2 Micro-Raman spectrum carried out on the LDG surface, 50x magnification, 514.5 nm laser source. The range 400-600 cm-1 is ascribed to bending in and between the SiO4 tetrahedra associated with cationic motions; the range near 800 cm-1 is the symmetric motion of adjacent Si atoms with respect to a bridging oxygen (Si-O-Si). The spectrum is consistent with the Libyan Desert Glass.

All the data collected were not compatible with a diagnosis of Heliodor, what the Laboratory found out is that this gem was a quite unknown and rare gem: it was a Libyan Desert Glass (LDG).

The Libyan Desert glass is a natural glass, but not so common as other natural glasses (as for example Obsidian), its origin is quite peculiar. It is a silica-reach natural glass found in an area of around 6500 km2 in Western Egypt, near the Libyan border and it was discovered in 1932. Since its discovery, the origin of this high silica content peculiar glass, is often discussed. LDG creation requires high temperature fusion process (temperatures above 1600°C to form and currently most of the scientists think that it probably originated by the impact of an extraterrestrial body into a silica sand or sandstone, fusing it. The subsequent solidification of this melted material produces the LDG [Aboud, 2009]. Other scientists believe that the melting process derived from a low altitude explosion of an extraterrestrial body, causing an airburst that melted the high silica content deposits.

In spite of its quite recent re-discovery,  LDG has really a long history. It formed 28.5 million years ago and humans appreciated it and used it from at least 3000 years [Kramers, et al., 2013]. It was used for example to carve a scarab beetle set in the central part of Tutankhamun’s pectoral [Fig. 3].

So, at the end, the client who submitted the gem to GIG was really surprised that his gem was not a Heliodor, but anyway happy to know he had a really uncommon and beautiful gem.

Fig. 3 Tutankhamun’s pectoral with a beautiful scarab beetle in LDG mounted in the central part of it, now at the Cairo Museum. [Image credit: Jon Bodsworth].

Find out more lab notes on issue 0 of Gulf Gemology Magazine

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