Thoughts on Sourcing New Zealand Greenstone
The "jade" artifacts found in New Zealand are nephrite, not jadeite. Nephrite is a calcium- and magnesium-rich mineral in the amphibole group. Specifically, it is a variety of actinolite. The other mineral commonly called "jade" is jadeite, which is a variety of pyroxene. Nephrite has been used as an ornamental stone by cultures around the world: China, the Pacific and Atlantic Coasts of North America, Europe, southeast Asia, and New Zealand. Nephrite is more common than jadeite and is usually found in low-grade, regionally metamorphosed rock.
Nephrite jade in New Zealand, usually called "greenstone" by locals, has been and continues to be important in Māori culture. The Māori fashioned both weapons (like the "mere" short club) and ornaments (like the "Hei-tikit" neck pendant) from nephrite. A variety of tools were also manufactured from the stone because metals were not available until the arrival of Europeans. Nephrite implements were valued and regarded as heirlooms.
The color of nephrite can vary widely. Many stones are various shades of green, but it can also be translucent white, opaque white, yellow, tan, and light gray. Microscopically, nephrite stones are comprised of compact, layered masses of long crystalline fibers. Other minerals can occur in stones that are principally nephrite. Diopside, epidote, plagioclase feldspar, and quartz are the most common while other minerals are less abundant.
Back in 2002 or so, while I was considering where I'd like to conduct archaeological studies, New Zealand was among the places that came to mind because it is such a beautiful environment. I started thinking about greenstone artifacts, looking into previous attempts to source them, and considering the difficulties encountered in those studies. Since then, one or two more successful sourcing studies have been conducted in other areas (i.e., Iizuka and Hung, 2005, "Archaeomineralogy of Taiwan Nephrite: Sourcing Study of Nephritic Artifacts from the Philippines," Journal of Austronesian Studies). In New Zealand, though, it seemed most attempts at sourcing had failed. The problem, from what I could gather, seemed to have been too much intra-source chemical variation. I couldn't find much information about these failed attempts -- no one seems to publish "failed" archaeological studies despite the importance of such data in other fields, like epidemiology. Anyway, I couldn't find much online besides vague mentions of doing studies, and my e-mails to a researcher interested in sourcing greenstone went unanswered.
While examining photos of New Zealand nephrite, like the one above, I noted that black "flecks" often occurred within it. These dark inclusions within the nephrite caught my attention. I knew that nephrite could have occasional inclusions of magnetite, pyrite, and such. If the nephrite itself or the material as a whole was highly variable within a single source, I wondered whether these inclusions could have unique chemical signatures and, therefore, would be useful for sourcing. This was similar to the approach I took to sourcing native copper.
The first step was identifying the dark inclusions. Were they magnetite? Pyrite? Something else? Actually the first step was obtaining pieces of New Zealand nephrite to analyze. This actually proved difficult. As I said earlier, a researcher who claimed to be interested in and working on sourcing greenstone did not answer my e-mails. There is also a complication that the Māori have declared greenstone to be taonga ("treasure"). Consequently, the material is protected by the Treaty of Waitangi, signed between the Māori and British in 1840. The government of New Zealand, therefore, restricts and carefully monitors the use of greenstone, and it is illegal to export raw greenstone. Instead of trying to obtain the necessary permits to export raw greenstone for study, I decided that I'd simply buy shaped pieces, like beads for jewelry. I then learned that most greenstone items sold in New Zealand are made of Chinese nephrite (sometimes made in China too)! It took me a while to find a source of the real thing.
When I examined the greenstone pieces using the electron microprobe, I found that these dark inclusions were primarily chromite, not magnetite or pyrite. Chromite, which is iron chromium oxide, is among the first minerals that crystallizes out of low-silica magmas deep within Earth's crust. Though chromite is igneous in origin, it is also found within metamorphic rocks because chromite withstands the high temperatures and pressures at which other minerals will alter and metamorphose. I suspect that this fact could make it very useful for sourcing greenstone. The fact that nephrite occurs in low-grade metamorphosed rock, in the other hand, might be linked to the intra-source variation. A variety of trace elements can occur in chromite, so those may yield a useful "fingerprint" for sourcing. Other minerals were also found in association with the chromite, and these, too, may be useful for distinguishing greenstone sources. Someday I'll have more time to investigate sourcing greenstone and see if this will indeed work...
Above: X-ray element maps (red and orange areas = high concentrations of that particular
element; blue and black = lowest concentrations); left = aluminum; center = calcium; right = iron
7/25/07
Added:
Electron Microprobe Analysis in Archaeology
Electron microprobe analysis (EMPA), also known as electron probe microanalysis (EPMA), is an analytical technique that combines scanning electron microscopy (SEM) and compositional analysis using x-ray spectrometry. The ability to determine structure and chemistry of samples makes EMPA very versatile. This is a dominant analytical technique in geology, but it is not as commonly used in archaeology despite similar materials in studied both fields. Here I will post about topics in EMPA, artifacts I have analyzed, archaeological studies that use EMPA, etc. If there is a topic you'd like to see posted here, please let me know.
Ellery Frahm
Doctoral Candidate, Archaeology
Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities
