Micro-World Gems & Gemology, Spring 2017, Vol. 53, No. 1

Ametrine Optical Dishes: Windows into the Effects of Crystal Structure

Bolivian ametrine crystal.
Figure 1. Left: An 85 × 25 mm ametrine crystal from Bolivia’s Anahí mine exhibits the deeply etched prism faces typical of this quartz variety. Right: The crystal’s termination, viewed in the direction of its c-axis, displays a rare “Y”-shaped manifestation of the crystal’s trigonal symmetry. Slices made perpendicular to this direction will reveal this symmetry with alternating amethyst and citrine sectors. Since the optic axis parallels the c-axis located at the intersection of the “Y” arms, all paths of light found across this termination face will be along the optic axis. Photos by Elise A. Skalwold.

During exploration of a gem’s interior, it is important to understand how crystal structure affects the incorporation and orientation of inclusions, how an inclusion can affect its host, how interpretation of observations can depend on point of view, and the nature of light as it passes through a crystal—host or inclusion. As proposed earlier in this column (Spring 2016, p. 81), polished inclusion study blocks are ideal for showcasing, studying, and reviewing basic concepts relating to the micro-world. Here we take a look at light and crystal structure.

Of all the varieties of quartz, ametrine is perhaps the most fascinating to observe. While its stunning color zoning makes it a popular designer gemstone, it also encompasses several optical properties unique to different quartz varieties that can all be explored in one specimen. Almost unrecognizable as quartz, ametrine crystals are deeply etched (figure 1, left) and have abundant healed fractures. The only clues to the trigonal symmetry typical of quartz are occasional rhombohedral faces and, very rarely, an enigmatic flat termination manifesting a three-armed figure (figure 1, right). Only when a basal section is made by cutting a slice perpendicular to the c-axis is the improbable yet beautiful combination of citrine and amethyst revealed.

Beauty and function are married in an ametrine study block oriented so that the view through its two largest faces is directly along the optic axis of a crystal, as seen in figure 1. The innovation of adding a set of concave dishes for sampling each color zone, as well as the borders between them, reveals truly remarkable optical phenomena (figures 2 and 3). These optical dishes allow the viewer to see light passing through the specimen in many directions, as does the strain-free glass sphere familiar to gemologists for use with a polariscope (figure 4). When viewed between crossed polarizing filters, these dishes dramatically display the different uniaxial optic figures unique to quartz, which vary depending on thickness and placement in the sectors: the classic “bull’s-eye” optic figure seen in the untwinned citrine sectors, an Airy’s spiral in the amethyst sectors, and a distorted figure produced in a border region. All of these observed phenomena are produced by optical activity (i.e., the rotatory dispersion of light as it travels along the optic axis direction within the quartz crystal). By analyzing these figures, we can determine handedness and the effects of thickness, while observations of the amethyst’s Brazil-law twinning may help distinguish natural versus synthetic origin.

Ametrine study block with concave dishes.
Figure 2. An ametrine study block with concave dishes cut and polished into its surface, including one on the opposite side. Viewed in transmitted light along the optic axis direction (i.e., all paths parallel to the c-axis), as in figure 1 (right), the trigonal symmetry of the crystal defines the citrine and amethyst sectoral zoning and allows observation of the effects of the optic axis on light. This specimen was fashioned by Nathan Renfro. Photo by Kevin Schumacher.
Bull’s-eye figures under crossed polarizing filters.
Figure 3. Placing the study block between crossed polarizing filters in transmitted light results in a variety of bull’s-eye optic figures that form in the divergent light produced by the concave dishes. The citrine sectors under the minor rhombohedral faces (z) are free of twinning and show a classic bull’s-eye optic figure with varying colors due to slightly different thicknesses. The amethyst sectors display an Airy’s spiral (middle row, right dish) produced by the alternating layers of left- and right-handed quartz of the Brazil-law twinning that formed under the crystal’s major rhombohedral faces (r); broadly angled colors in these latter areas are also indicative of this twinning. The dish on the upper right is a distorted figure on the border of the sectors. Photo by Kevin Schumacher.
Illustrations of light rays.
Figure 4. Left: A section through part of the study block. The dish serves as a concave (negative) lens that causes the light rays to diverge. Right: A gemologist’s strain-free glass sphere is used to observe an interference figure. Although the rays first converge at the base of the sphere, they ultimately diverge just like the rays in the figure on the left. Illustration by William A. Bassett.

For an in-depth analysis of the phenomenon of optical activity and the origin of quartz’s optic figure, see E.A. Skalwold and W.A. Bassett, Quartz: A Bull’s Eye on Optical Activity, Mineralogical Society of America, 2016, www.minsocam.org/msa/OpenAccess_publications

Elise A. Skalwold, an accredited senior gemologist of the Accredited Gemologists Association (AGA), is involved in curating and research at Cornell University in Ithaca, New York. William Bassett is a research scientist and professor emeritus of geology at Cornell University.