Blasting gems with lasers to determine their origins

Precious stones are an enduring fascination for mankind and for centuries they've been traded and given as gifts, either as polished stones or embedded in jewelery. In so doing, they have traversed the planet, sometimes ending far from the source from which they were mined. Apparently a gem's origin is not only a matter of curiosity but can in fact have a significant bearing on its value. Scientists from New Mexico State University in the US have proposed a way of extracting information about a gem's origin from its chemical composition by using laser-induced breakdown spectroscopy (LIBS)¹.

Laser-induced breakdown spectroscopy is a way of measuring the optical signature of a sample by heating it to high temperature so that the individual atoms emit light at discrete wavelengths. Atoms emit discrete wavelengths of light which are unique identifiers of that particular atom, whether it be gold, silicon or iron, for example. It is therefore possible to discern the chemical composition of a sample by identifying which wavelengths appear in the emitted light spectrum. The beauty of using a laser to do the heating is that it can produce the necessary temperatures in very small samples. Because very little material is vaporised, LIBS is often referred to as being minimally destructive.

The researchers used a Nd:YAG laser operating at its fundamental wavelength of 1064 nm in the near infra-red. A single 7 ns pulse from the laser, with an energy of approximately 102 mJ, was focused down on to and absorbed by the sample, producing a micro-plasma at its surface and resulting in a crater 300 to 400 μm in diameter. Using a lens to project the light from the plasma onto a bundle of optical fibres, the light was then relayed to a series of seven spectrometers, which combined to give a spectrum of the plasma light ranging from 200 nm in the ultraviolet, spanning the entire visible spectrum and ending in the near infra-red at 980 nm. The discrete wavelengths indicating the presence of given elements were most prominent during the cooling stages of the plasma, revealing information about the chemical composition of the small volume of vaporised material.

Statistical analysis was conducted on the LIBS spectra, resulting in some limited success and making it possible to distinguish between gems from different rock types in the same mine. However it was not possible to distinguish between those from mines in different regions altogether. Because of the complexity of the stones' chemical make-up, there was no way of classifying them according to their sources. The spectroscopic technique could be said to provide too much information, making the data hard to interpret.

The researchers propose that, for the technique to work in practice, there would need to be a vast database containing the signatures of thousands of gems, from all possible known origins. This does not currently exist and, to build such a database, the owners of these precious gem stones would have to be willing to part with them and have them subject to some minor damage by vaporisation. Some of them might just frown upon that.

1) C. McManus, N. McMillan, R. Harmon, R. Whitmore, F. DeLucia, and A. Miziolek, "Use of laser induced breakdown spectroscopy in the determination of gem provenance: beryls," Appl. Opt. 47, G72-G79 (2008). DOI: 10.1364/AO.47.000G72