Star Topaz from Vietnam
In addition to ruby, sapphire, spinel, tourmaline, garnet, and aquamarine, Vietnam is a source of topaz, discovered in many mining areas of several northern and Central Highlands provinces. These crystals are mainly colorless or very light to light blue. In 2020, we purchased a star topaz, reported to be from Lam Dong, Vietnam (figure 1). It was cut and sold by a gem setter in Ho Chi Minh City.
This transparent oval cabochon weighed 271.76 ct and measured 43.98 × 32.16 mm and 19.12 mm high. Observed with the unaided eye under a spotlight, it had a visible eight-rayed star effect. Testing at Liu Gemological Research and Application Center revealed the following standard gemological properties: refractive index—1.620–1.627 (measured on the flat polished bottom); hydrostatic specific gravity—3.56; biaxial optic figure under the polariscope; inert to long- and short-wave UV radiation; very weak pleochroism (colorless to light blue); and two-phase inclusions and needle-like inclusions. These characteristics confirmed a natural topaz.
We directed a flashlight as reflected light on the top of the stone, keeping the background dark to accentuate the phenomenon. An eight-rayed asterism appeared, caused by a strong band of light that extended along the length of the stone. This band of light formed the two main arms of the star. Six sharper arms extended from the main band. These six arms were formed by three bands—one perpendicular to the main light band and the other two inclined to it. Depending on the stone’s orientation, the arms of the inclined light bands were split into two parts or crossed the intersection of the main light band and the light band perpendicular to it. Each pair of inclined arms formed an angle of ~98° (i.e., the single light band was inclined at ~49° to the main light band). The length and width of the rays changed when we directed the light at different angles (figure 1).
Under the microscope, we recorded white needle-like inclusions but could not identify them. They were oriented in four directions, and this was responsible for the eight-rayed star effect. These inclusions did not form long needles but slightly elongated dashes that were unevenly distributed in the stone (figure 2). This uneven distribution of inclusions explains why some rays became blurred or vanished while others were continuous and sharp.
(001) plane, the needle-like inclusions aligned in four directions to form an
eight-rayed asterism (right).
Topaz has been documented to have chatoyancy, four-rayed asterism (Spring 2007 GNI, p. 73; Summer 2008 GNI, pp. 182–183), and six-rayed asterism (M. Steinbach, Asterism—Gems with a Star, MPS Publishing and Media, Idar-Oberstein, Germany, 2016). However, there have been no reports of topaz with eight-rayed asterism. The orthorhombic system has four Bravais lattices: primitive, base-centered, body-centered, and face-centered. When the topaz crystal has a base-centered lattice, the needle-like inclusion will align to four directions on the (001) plane: two perpendicular directions of a and b, and two diagonal directions intersected at the center of the top and bottom faces of the lattice. These directions are perpendicular to the c-axis (figure 3). When the light reflects off these inclusions, an eight-rayed asterism will appear. A typical example of an orthorhombic mineral with an eight-rayed asterism was bronzite, introduced by Steinbach (2016). Nevertheless, this eight-rayed star topaz was a rare and intriguing case.
