Characterisation of New Zealand nephrite for forensic purposes

Abstract

This study investigated the discrimination between sources of the semi-precious mineral, nephrite, by a targeted microanalytical determination of the elemental composition, including the trace elements. Nephrite specimens were collected from two significant nephrite sources in New Zealand, namely the Westland and Wakatipu fields, and combined with donated specimens from the Southland field to complete a representative collection of New Zealand nephrite. A small number of nephrite specimens were donated from the South Westland nephrite field and from foreign sources. Representative fragments of these specimens were analysed by electron microprobe analysis (EMPA) for major elements and by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for trace elements. The data obtained by the analytical procedure were treated within a compositional data (CoDa) framework of statistical analysis that focuses on the relative sizes of the components in the data set. The data were transformed under the guidelines of the CoDa framework, where appropriate, and the transformed data were treated with standard statistical methods for exploratory data analysis, dimension reduction and discriminant analysis. Advances were made to the Hotelling’s method for comparison of multivariate means by incorporating a permutation evaluation step. This permutation method removes the requirement for multivariate normality, and it also allows comparisons to be made when there are many more variables than observations, as is often the case when objects are being characterized using elemental data. The strategy used in this study showed that it is possible to discriminate between sources of nephrite at both an intra- and inter-source level within New Zealand. In addition, an exploratory investigation showed that New Zealand nephrite could be differentiated from the few nephrite specimens from foreign sources that were available for comparison. Recommendations are made for the protection of the New Zealand nephrite resource and for casework, based on the results obtained in this study.