Micro-World Gems & Gemology, Fall 2019, Vol. 55, No. 3

Tourmaline Inclusion in Russian Alexandrite

Well-formed prismatic tourmaline inclusion in Russian alexandrite.
The longest inclusion in this photomicrograph of a Russian alexandrite was conclusively identified as tourmaline. This tourmaline inclusion exhibits a well-formed prismatic/rod-like crystal shape. Darkfield illumination. Photomicrograph by Jonathan Muyal; field of view 1.44 mm.

Alexandrite is the color-change variety of the mineral chrysoberyl. The most coveted alexandrite exhibits a luscious green to greenish blue color in daylight and a warm raspberry red in incandescent light. This phenomenal color change is caused by the presence of trace Cr3+ substituting for Al3+ in the chrysoberyl crystal structure. Alexandrite was originally found in the Ural Mountains of Russia in the early nineteenth century and was named in honor of Czar Alexander II.

GIA recently examined several Russian alexandrite rough specimens. These were reportedly from the Tokovaya mining area in the Ural Mountains, located a few kilometers away from the city of Yekaterinburg. Closer microscopic examination revealed a brown, transparent, well-formed prismatic crystal inclusion with triangular termination, indicating it might belong to the trigonal crystal system (see above).

The inclusion proved to be doubly refractive when viewed between crossed polarizers. Raman microspectrometry analysis conclusively identified the inclusion as tourmaline. In order to validate the Raman results and further identify the tourmaline species, laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) was used to obtain the chemistry of the inclusion (Z. Sun et al., “A new method for determining gem tourmaline species by LA-ICP-MS,” Spring 2019 G&G, pp. 2–17). It was classified as dravite, a sodium- and magnesium-rich tourmaline species. The trace element chemistry of the host alexandrite matched well with GIA’s Russian alexandrite chemistry reference data.

Inclusions in alexandrite from various localities have been reported (E.J. Gübelin and J.I. Koivula, Photoatlas of Inclusions in Gemstones, Vol. 1, ABC Edition, Zurich, 1986, pp. 265–267; Vol. 3, Opinio Publishers, Basel, Switzerland, 2008, pp. 372–405). However, tourmaline in an alexandrite host is rare.

Alexandrite is traditionally popular for its beautiful color-change phenomenon rather than its inclusion scenes. The fine tourmaline crystal inclusion in this Russian alexandrite makes it an unusual collector’s gem specimen.

Jonathan Muyal is a former staff gemologist, and Ziyin Sun is a research associate, at GIA in Carlsbad, California.