Journal Digest: Radiocarbon Age Dating on Natural Pearls

Guy Lalous ACAM EG is on-hand to summarise some of the more in-depth articles from Gem-A's The Journal of Gemmology. Here, he explores an article on radiocarbon age dating of natural pearls from the Winter 2017 issue. 

This article describes how radiocarbon age dating can be adapted to the testing of historic pearls. The authors, Michael S. Krzemnicki, Laurent E. Cartier and Irka Hajdas, have developed their sampling method so that radiocarbon age dating can be considered as quasi non-destructive. The refined sam­pling process allows researchers to work with tiny amounts of nacre powder (~2 mg) taken from a drill hole without any damage to the outer surface of a pearl. In this article, pearls originating from a historic shipwreck were submitted to radiocarbon age dating. 

A small selection of pearls (approximately 2–8 mm diameter) from the Cirebon shipwreck was investigated for this study.
The pearls are shown on a historic map of the Java Sea, where the shipwreck was discovered. Photo by Luc Phan, SSEF.

The 10th century Nan-Han shipwreck was discovered accidentally in 2003 off the northern coast of Java, Indone­sia, near the city of Cirebon. The excavation of the ancient merchant vessels produced Yue ceramics, glassware and Chinese coins dating from the 10th century CE, jewellery, loose gemstones and also a number of carved gastropod shells and pearls. The thousands of glass fragments, and several unbroken blue and green glass objects found in the Cirebon shipwreck, undoubtedly originated from the Islamic Middle East. 

This indicates extensive trade in Southeast Asia along maritime routes at that time, which the Cirebon merchant vessel was a part of. This also supports a Persian Gulf origin for the pearls. The partly abraded and brown-to-grey altera­tions around the drill holes of these pearls suggests that they might have been in use for some time, strung on strands or set with metal lin­ings in jewellery before they sank in the vessel with the rest of its cargo. The coins and artefacts provided good evidence for a 10th century age of the shipwreck. 

What about X-radiography?

X-radiography is an imaging technique. X-rays are located beyond UV in the electromagnetic spectrum, where they have even shorter wavelengths and greater energy. Materials of low atomic weight allow x-rays to pass through easily and, therefore, appear dark on x-ray film, and those of high atomic weight block x-rays and appear white.

What about X-ray luminescence Computed Tomography?

X-ray luminescence is an emerging technology in X-ray imaging that provides functional and molecular imaging capability. This emission-type tomographic imaging modality uses external X-rays to stimulate secondary emissions, which are then acquired for tomographic reconstruction. This modality surpasses the limits of sensitivity in current X-ray imaging. 

What about EDXRF?

X-Ray fluorescence analysis using ED-XRF spectrometers is a commonly used technique for the identification and quantification of elements in a substance.

What about X-ray computed microtomography?

Seeing inside a material object, in three dimensions, is often crucial for proper characterisation, so that the link between microstructure and properties can be made. Micro-computed tomography or "micro-CT" is X-ray imaging in 3D on a small scale with massively increased resolution. It really represents 3D microscopy, where a very fine scale internal structure of an object is imaged non-destructively.

Fourteen pearls were analysed routinely by X-radiography and X-ray lumines­cence, as well as by EDXRF spectroscopy. Four of them were selected for X-ray computed microtomography (micro-CT) analysis. 

These 14 pearl samples (71742_A–N; approximately 2–8 mm diameter) from the Cirebon shipwreck were examined for this study.
They are partially abraded around their drill holes and show some brown to grey colour alterations. Photo by Luc Phan, SSEF.

Based on their X-radiographs, trace-element com­position and lack of luminescence to X-rays, the samples studied for this report were all saltwater natural pearls. The separation of natural from cultured pearls is mainly based on the interpretation of their internal structures. The radiography and micro-CT scans revealed that their internal structure main­ly consisted of fine ring structures typical of natu­ral pearls.

What about radiocarbon age dating?

The collision of high-speed neutrons produced by cosmic ra­diation with the nucleus of nitrogen results in the capture of a neutron and the expulsion of a pro­ton, thus transforming the 14N isotope into the radionuclide 14C. The radiocarbon, present only in trace amounts in the atmosphere, combines with atmos­pheric oxygen and forms radioactive carbon di­oxide, which is then incorporated into plants by photosynthesis and subsequently into animals via respiratory and metabolic pathways. As a consequence, the radiogenic 14C is incorporated into the endo- or exoskeletons (e.g. bones or shell structures) of animals.

After death, the lifelong exchange of carbon with the environment suddenly stops, resulting in a slow radioactive decay of 14C in the dead plants and animals. By measuring the ratio of radiogen­ic and stable carbon isotopes (14C/12C), it is thus possible to determine their age. The so-called half-life of 14C (that is, the time at which only half of the original 14C is still present in a sample and, as such, represents the constant rate of de­cay over time) is about 5,700 ± 40 years.

What about MICADAS?

The MICADAS is a mini carbon dating system through accelerator mass spectrometry. It is a two-step process. The first part involves accelerating the ions to extraordinarily high kinetic energies, and the following step involves mass analysis. The system allows radiocarbon analyses of ultra-small samples with great accuracy in only a couple of hours’ time. 

A pearl is a calcium carbonate (CaCO3) con­cretion formed by bio mineralisation processes in a mollusc—very much the same processes as for shell (exoskeleton) formation. As such, pearls (and shells) contain carbon, mainly the stable iso­tope 12C (as well as 13C) but also a small fraction of radiogenic 14C. The carbon used for the bio mineralisation of pearls and shells mainly originates from two very different carbon pools: (1) oceanic dissolved inorganic carbon; and (2) respiratory CO2, mainly stemming from food metabolism.

As such, the so-called marine reser­voir age effect may distinctly affect the resulting 14C ages of shells and pearls, especially in areas with upwelling of ‘old’ water.  Hence, a correction is required to take into account the geograph­ic location of the sample. The 14C/12C ratio was measured on three samples using the Mini Carbon Dating System (MICADAS).  

The calculated 14C age BP was corrected by apply­ing a marine reservoir correction that was based on values for the Java Sea location. These were estimated (mean-weighted) based on 10 data points in the vicinity of the sampling site. The result corresponds approximately to the end of the 10th century, which cor­relates well with the age stipulated for the coins, pottery and other artefacts found in the shipwreck. 

 Age determination can support evidence of historic provenance in the case of antique jewellery and iconic natural pearls. It can also be used to identify fraud in cases where, for example, younger pearls are mounted in historical jewellery items, or have been treated so that they appear older rather than having been farmed during the 20th century. 14C age dating can be used to obtain evidence to support a decision whether a pearl is of natural or cultured formation. This is because methods to commercially cultivate pearls from certain mollusc species only began during the 20th century.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled ‘Radiocarbon Age Dating of 1,000-Year-Old Pearls from the Cirebon Shipwreck (Java, Indonesia) by Michael S. Krzemnicki, Laurent E. Cartier and Irka Hajdas 2017/Volume 35/ No. 8 pp. 728-736

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