Micro-World Gems & Gemology, Winter 2023, Vol. 59, No. 4

Quarterly Crystal: Hiddenite from Idaho

John Koivula, Nathan Renfro, and Maxwell Hain

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Figure 1. Weighing 20.57 ct and measuring 37.10 mm in length, this doubly terminated, heavily etched spodumene crystal contains chromium as its coloring agent or chromophore. Photo by Adriana Robinson.
 
Figure 1. Weighing 20.57 ct and measuring 37.10 mm in length, this doubly terminated, heavily etched spodumene crystal contains chromium as its coloring agent or chromophore. Photo by Adriana Robinson.

Not every day do we encounter a known but rare gem mineral from a previously unknown locality for the species. But that is exactly what happened when we examined the doubly terminated, heavily etched green crystal shown in figure 1. This slightly yellowish green crystal, weighing 20.57 ct and measuring 37.10 × 10.00 × 8.50 mm, is from Sawtooth National Forest in Elmore County, Idaho. It came to us from the mineral collection of Jack Lowell of Tempe, Arizona.

Figure 2. Found on the surface of the heavily etched crystal, these sail-shaped etch features captured with differential interference contrast photomicrography provide definitive proof of the mineral spodumene. Photomicrograph by Nathan Renfro; field of view 1.44 mm.
 
Figure 2. Found on the surface of the heavily etched crystal, these sail-shaped etch features captured with differential interference contrast photomicrography provide definitive proof of the mineral spodumene. Photomicrograph by Nathan Renfro; field of view 1.44 mm.

Mined by Geary Murdoch in the 1990s, the crystal was sold to Lowell as an etched aquamarine. Lowell suspected that it was a spodumene crystal due to the very distinctive and diagnostic etching on its surfaces (figure 2). The authors examined the crystal with Raman analysis to confirm its identity as spodumene. Energy-dispersive X-ray fluorescence identified its chromophore as chromium, indicating the specimen was the rare green variety known as hiddenite.

Figure 3. This 2.30 ct doubly terminated hiddenite crystal comes from the Adams mine in North Carolina, the first known locality for the gem. Photo by Annie Haynes.
 
Figure 3. This 2.30 ct doubly terminated hiddenite crystal comes from the Adams mine in North Carolina, the first known locality for the gem. Photo by Annie Haynes.

Idaho is only the second known hiddenite source. The first to yield gem crystals (figure 3) was the Adams hiddenite and emerald mine in Alexander County, North Carolina.

John Koivula is analytical microscopist, Nathan Renfro is senior manager of colored stone identification, and Maxwell Hain is a staff gemologist, at GIA in Carlsbad, California.