Microstructural controls on the quality and fracture toughness of New Zealand nephrite jade (pounamu)

Nephrite jade is a monomineralic rock composed of fine-grained interwoven amphibole fibres of the tremolite-actinolite series. This rock has a great cultural importance for many countries due to its combination of beauty, toughness, and high fracture resistance. In New Zealand, nephrite jade (pounamu) is culturally significant to Māori, with a documented history of use in the manufacture of tools, weapons, jewellery, and talismans. Despite this historical importance, there is a lack of systematic quantitative data on the physical and mechanical properties that contribute to its workability and durability.Traditionally nephrite jade’s suitability for carving has been relying on carvers’ expertise and empirical knowledge. Drawing on the collective experience of Māori artisans, a preliminary survey of pounamu jade carvers (n=20) ranked the quality of a collection of specimens, showing positive correlation between perception of material quality and measured anisotropy in compressional (P-) wave velocity. Current research aims to evaluate the intrinsic origin of this anisotropy and it’s link to mechanical properties through combined microstructural and mechanical analysis.Electron backscatter diffraction (EBSD) maps of three samples characterised microstructural patterns associated with perceived quality. A sample subjectively rated by carvers as high-quality is noted for homogeneous microstructure with a relatively small grain size (Equivalent diameter on average