2026

Age dating quality control

By Dr. Markus Wälle, first published in Facette 30 (March 2026)

Knowing the age of a gemstone can be valuable when determining its origins. Unfortunately, most gemstones cannot be dated directly. However, it is possible to determine the age of certain inclusions commonly found in gemstones. At SSEF, zircon inclusions in sapphires or rubies (and other gems) are regularly used to determine the stone’s age, provided the inclusions are exposed on its surface. Zircon is a robust mineral that can survive high temperatures and strong acids. It incorporates uranium (U) and thorium (Th) but not lead (Pb) as it grows. These two elements decay over time into different lead isotopes. Three independent ages can be derived from zircon age determinations. These are the 206Pb/238U, 207Pb/235U and 208Pb/232Th ages. The latter is normally not taken into account as Th tends to be present in lower concentrations than U. This, together with Th’s longer half-life, leads to higher uncertainties in the 208Pb/232Th age. When all three ages are in agreement with each other (within uncertainties), we can be confident that an accurate radiometric age has been measured for the inclusion. This establishes an upper age limit for the host gemstone, since a crystal inclusion cannot be younger than its host.
Figure 1: Shows a Concordia plot of our results and the reference value. The numbers represent ages in millions of years.

Zircon is a relatively common mineral which is very suitable for dating due to its robustness, lack of initial lead and adequate uranium concentrations. It has therefore become a staple in the field of geochronology, with several laser ablation ICP-MS facilities in earth sciences departments all over the world dating thousands of zircon grains per year.

A few years ago, the International Association of Geoanalysts (IAG) introduced a quality control programme for these laboratories, involving the distribution of identical samples to all participating laboratories as a blind-test. Several months after all participating labs have resubmitted their sample and their age results, the IAG publishes a report with the anonymised results from the participants and an independently determined ‘reference’ age established by TIMS (thermal ionisation mass spectrometry), a much more time-consuming but also more accurate method than LAICPMS. It’s already several years, since SSEF is taking part in this quality control programme.

For the most recent (third) testing round in 2024 the IAG sent out zircon samples known as Peng-16, named after the Pengli Quarry in China’s Guangxi Province, where they were collected in 2016. After analysing the Peng-16 sample at SSEF meticulously we calculated a Concordia age of 150 ± 4 Ma for the zircon, calculated from 30 measurements performed on three separate occasions, with uncertainties at the 95% confidence level.

At the end of 2024 the IAG finally released the results of the most recent testing round. The TIMS analysis yielded a 206Pb/238U reference age of 159.27 ± 0.13 Ma. Figure 1 shows our result and the reference age in a Concordia plot, where the two different U-Pb decay systems are plotted on orthogonal axes and the blue line indicates where they agree with each other. As in the past quality control rounds, the accuracy of our radiometric age dating was confirmed by this testing.