Winter 2009

Uranium Mineral as Inclusions in Turquoise

Client requests to identify turquoise are relatively common at the GIA Laboratory, and there are seldom any surprises. Recently, however, turquoise beads and cabochons submitted by several different clients revealed some very unusual inclusions (figure 1). The source of the turquoise was stated to be China's Hubei Province.

Chinese turquoise is well known, but we were surprised to see bright yellow inclusions, identified by Raman analysis as francevillite [(Ba, Pb)(UO₂)₂V₂O₈•5H₂O], a hydrated barium uranium vanadate. Francevillite forms a solid-solution series with curienite [Pb(UO₂)₂V₂O₈•5H₂O], so a mixture of both could be present, though our Raman analysis clearly pointed to francevillite as the dominant mineral. Occurring as concretions with a radial habit, as circular to semicircular rings, and as nondescript masses and seam fillings (see, e.g., figure 2), these yellow inclusions stand out in sharp contrast with the blue background of their host. These francevillite inclusions were so obvious, sometimes even without magnification, that we are certain no such turquoise has entered our laboratory before. This suggests that a new find of turquoise is being mined.

Because uranium is a major constituent in both francevillite and curienite, we decided to check these turquoise samples for radioactivity with a Geiger counter. We thought they might register a readable signal, in the same way we can detect low levels of natural radiation in gems such as ekanite or zircon with this instrument. Our assumption proved to be correct: Very low levels of radiation were detected, supporting our Raman identification of this previously unknown inclusion in turquoise.

To detect radiation in gem materials, the GIA laboratory uses Victoreen 290 survey meters with so-called pancake probes, which are calibrated on an annual basis. To test an item, we first determine the current background radiation level (i.e., that present in our everyday environment from cosmic rays, radon, and other sources) and then check for radioactivity above that level in the gem material. Most of the turquoise samples with francevillite were only slightly above background. A few showed more elevated levels, reaching up to 7 or 8 millirems/hour, which is still considered very weak and generally harmless. By comparison, orange glazed Fiestaware ceramics, which have long been known to be somewhat radioactive, measure ~40 millirems/hour.

Nature surrounds us with radiation on a daily basis, coloring many objects in our lives. Gems such as pink tourmaline, green diamonds, and smoky quartz all derive their color from irradiation (see C. E. Ashbaugh, “Gemstone irradiation and radioactivity,” Winter 1988 G&G, pp. 196–213). While turquoise gets its blue bodycolor from copper, the bright yellow francevillite inclusions add an interesting aspect to their host.

It is important to note that although exposure to these levels of radiation is not considered significant, breathing dust that might be created during cutting should be avoided. For more information concerning radioactive minerals or radiation in general, we suggest contacting specialists in the field, such as the Health Physics Society (www.hps.org). In circumstances such as this, GIA Laboratory reports contain a comment stating “A naturally occurring accessory mineral is present that emits radiation detectable with a standard survey meter.”

John Koivula and Shane McClure