Gem News International Gems & Gemology, Spring 2021, Vol. 57, No. 1

Phenakite as a Diamond Imitation

Jaroslav Hyršl

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Suite of phenakite ranging from 0.20 to 0.98 ct.
Figure 1. A set of phenakite gems ranging from 0.20 to 0.98 ct, with a total weight of 5.85 carats, sold as diamonds. Photo by J. Hyršl
In my gemological laboratory, I see some interesting imitations from time to time. The latest was quite unusual: a set of 12 loose stones submitted as diamond (figure 1) for certification. Testing their thermal conductivity with a Presidium Gem Tester and their electrical conductivity with a Ceres Secure II device indicated diamond and ruled out synthetic moissanite. The next property examined was fluorescence, and all the stones were completely inert to both short-wave and long-wave UV. It would be unusual for a group of 12 natural diamonds to all be inert, so the next step was determining whether they might be laboratory-grown diamonds. Working under the assumption that they were indeed diamonds as submitted, the stones were tested for short-wave UV transparency and all were very transparent, suggesting they could be type IIa (which would be consistent with laboratory-grown near-colorless diamonds). Finally, I performed Raman analysis; instead of a sharp diamond peak at 1332 cm–1, it showed only a small peak at 878 cm–1, indicating that these stones were not diamonds.

Only then did I remember a natural crystal of phenakite, sold as diamond, that I tested a few years ago. It was very similar to natural rough diamond, but instead of typical trigons, it had a surface covered by many rhombs. But its thermal conductivity was the same as diamond’s, and therefore it was a plausible diamond imitation for a mineral or gem dealer without knowledge of crystallography. When I remembered this, the identification was easy—the refractive index of the loose cut stones was 1.655–1.670, and the Raman line matched very well with phenakite. And upon further examination, the stones appeared doubly refractive in the microscope and polariscope.

Phenakite is trigonal Be2SiO4, colorless, and similar to beryl. Normally it forms hexagonal prismatic crystals that look very different from diamond. Nevertheless, some crystals from Nigeria are irregularly developed (figure 2) and could be mistaken for a rough diamond without careful examination, especially if mixed with real diamonds.

Nigerian phenakite crystal.
Figure 2. Natural phenakite crystal from Nigeria, width 38 mm. Photo by J. Hyršl.
Cut stones are quite rare and well known only to collectors of rare gemstones. Distinguishing a cut phenakite from diamond is quite easy when one can measure refractive index, see a double refraction in the polariscope or microscope, or calculate weight from measurements. Much more difficult would be phenakite set in jewelry, where it can be very difficult to measure RI. Cut phenakite has much lower brilliance than diamond, of course, but can resemble low-quality cut diamonds.

Jaroslav Hyršl (hyrsl@hotmail.com) is a mineralogist and author in Prague.