Micro-World Gems & Gemology, Summer 2020, Vol. 56, No. 2

Böhmite in Corundum

Richard W. Hughes

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Böhmite fills a cavity in the ruby.
A large area of a foreign substance cut through on the surface of a ruby from the Montepuez area of Mozambique. This substance infilled not only this cavity but also many narrow fissures in the stone. Micro-Raman analysis identified it as böhmite. Darkfield + diffuse overhead illumination. Photomicrograph by Richard W. Hughes; field of view 3 mm.

Gemologists at Bangkok’s Lotus Gemology laboratory recently examined a large lot of rubies believed to originate from the Montepuez area of Mozambique. Microscopic examination confirmed that assessment. Many of these stones displayed features in their infrared spectra that are typical for böhmite, with a characteristic “twin peak” spectrum at ~3309 and 3089 cm–1 (figure 1).

Infrared spectrum of böhmite in ruby from Mozambique.
 
Figure 1. “Twin peak” infrared spectrum of böhmite in the Mozambique ruby, with prominent peaks at ~3309 and 3089 cm–1.

One of the stones showed a large area cut through on a pavilion facet that appeared to be a foreign substance. A similar substance was seen in many of the fissures in the stone (top of page). Judicious use of the hot point under the microscope produced no reaction. We then moved to analyze the substance via micro-Raman. The result was böhmite, which neatly confirmed the results from the infrared spectrum.

Böhmite, AlO(OH), is a polymorph of diaspore. In corundum it is generally a secondary mineral, infilling surface-reaching openings after the host corundum has grown.

Richard W. Hughes is an award-winning gemologist and the co-founder of Lotus Gemology in Bangkok.