Prismatic Rutile in Quartz
Rutile, a mineral composed mainly of titanium dioxide (TiO₂), is a common inclusion in quartz in the form of profuse acicular hair-like crystals (E.J. Gübelin and J.I. Koivula, Photoatlas of Inclusions in Gemstones, Vol. 1, ABC Edition, Zurich, 1986). These golden needle inclusions are appreciated as a pleasing feature, giving their name to the variety known as rutilated quartz.
However, prismatic single rutile crystal inclusions in quartz remain an underappreciated feature, often synonymous with “flaws.” They are rarely showcased by the lapidary. Nevertheless, we observed a 21.27 ct cushion-cut rock crystal quartz that displayed a large, well-formed rutile crystal inclusion (see above) under the table.
The protogenetic inclusion showed a black-silver color with adamantine to submetallic luster, a well-formed tetragonal (“stubby”/blocky) prismatic crystal habit with fine striation along its length (parallel to the c-axis), and smooth pyramidal termination faces. Cyclical twinning such as twin knee/sharply angled twins on  and parallel twinning crystal growth along the length were also observed (again, see above).
Very close below, in the background, was another rutile crystal inclusion, this one a fragmented piece. At first glance, it could be mistaken for part of the main crystal inclusion described above. Nevertheless, this rutile fragment adds details to the overall visual composition.
Rheinberg illumination (Fall 2015 Micro-World, pp. 328–329) using blue and yellow filters provided additional contrast. Lighting technique is critical in photomicrography. Here it dramatically enhanced the inclusion scene, offering alternative vibrant colors for aesthetic purposes.
This rock crystal quartz had preserved and beautifully highlighted a prismatic rutile crystal inclusion, like a collector mineral specimen in a display window. Such a large inclusion specimen also provides valuable mineralogical information for the gemologist.