PROVENANCE OF RUBIES AND SAPPHIRES:
AN APPLICATION OF LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS)
AND ADVANCED CHEMOMETRICS FOR THE GEM INDUSTRY


Rubies and sapphires totaling 572 samples of known origin from 24 deposits in 12 countries (Afghanistan, Australia, Greenland, India, Kenya, Madagascar, Myanmar, Pakistan, Sri Lanka, Tanzania, the United States, and Vietnam) were analyzed using an Ocean Optics 2500+ LIBS system. A 1064 nm Nd:YAG nanosecond laser was focused on a small area of each stone and pulsed at 120 mJ to ablate the sample, forming a short-lived, high temperature plasma. As exited electrons decayed back to their stable configurations, photons were emitted and transmitted via an optic fiber to a spectrometer that separated the light into its constituent wavelengths, each of which corresponds to a specific element.  Analyses were performed in an argon atmosphere to enhance the signal and minimize traces of testing. Average ablation craters were approximately 200µm diameter; LIBS systems can be customized to minimize damage. Each stone was analyzed 30 times. Raw spectra were processed with a pair of advanced chemometric techniques: principal component analysis (PCA) and partial least squares (PLS) regression using Unscrambler software. PCA was used to simplify the dataset from 13,700 dimensions (each wavelength measured) to two to four dimensions by generating meaningful combinations of wavelengths (elements).

 

PLS regression models were calculated to characterize half the sample set (trainers). The trainers defined chemofacies, or chemically distinct datasets, which could be separated using the algorithm developed in this study. Because each deposit defines a unique chemofacies, it is possible to predict the provenance of the samples not used in the calculations (verification samples). Using the algorithms developed, tests of 122 verification sapphires and 164 verification rubies yielded correct identification rates of 93% and 90% respectively. Misidentified samples were the result of poorly characterized chemofacies due to small sample sets. This work demonstrates that the provenance of rubies and sapphires can be determined through chemometric analysis of LIBS spectra.


Date Presented:  May, 2011
Conference Presented: GIA Symposium 2011
 

Authored By:
1. Kristen Yetter, Nancy J. McMillan
2. Catherine McManus, Tristan Likes
 

Author Affiliations:
1.  Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico
2.  Materialytics, Harker Heights, Texas