THE HAMLIN NECKLACE AND MOUNT MICA TOURMALINES

The Hamlin Necklace is a unique and innovatively made example of nineteenth-century North American jewelry. This treasure of the Mineralogical and Geological Museum at Harvard University consists of eighteen removable gem pendants, mainly containing tourmalines from Mount Mica, Maine. The first North American gem quality tourmalines were discovered and mined at Mount Mica in 1820 by Augustus C. Hamlin (1829-1905), who also designed and bequeathed the necklace along with a set of tourmaline gemstones to Harvard University in 1934. Tourmaline is well known for its complex structure and chemical variety, with attractive gems being highly desirable. The aim of this study is two fold: 1) to present a detailed description and history of the Hamlin Necklace, and 2) to study this unique collection of gem tourmalines and set gemstones in the Hamlin Necklace by Raman spectroscopy and LA-ICMPS.

The Hamlin Necklace tourmalines can be classified as Elbaite; according to the Raman spectra of the metal ion-oxygen bond vibrations, wavenumbers below 1600 cm^-1, Elbaite is characterized by sharp peaks at 222 ± 2, 635 ± 2, 708 ± 2, corresponding to Mg-O, Fe-O stretching bond, Si-O bridging oxygen in the Si-O rings, and Si-O-Si bend, respectively. The OH₁ stretching vibrations, located at the center of the hexagonal rings, and the OH₃ stretching vibrations located at the border of the hexagonal columns, can be found at 3653 ± 2 and < 3600 cm^-1, respectively. Further considerations are the relationship between the intensity ratio of the OH Raman peaks versus trace element concentrations, specifically those on the X-site (Sr, Ba, and Eu²⁺), and correlations observed with the alkali contents and the X-site.

This novel approach of non-destructive techniques, spectroscopy as well as major and trace element analysis, allows us to determine the chemical distribution and trace element patterns in tourmaline, while establishing and further correlating the preferential presence of specific trace elements in the X-site with vibrational energies of water molecules. This dual-technique has great potential to be applied to other gemological materials to distinguish provenance, natural versus synthetic materials and treatments.