实验室注意事项 Gems & Gemology, Spring 2013, Vol. 49, No. 1

Buff-Top Round Diamond


实验室注意事项
Figure 1. A profile view of this buff-top round diamond shows its low polished dome. Photo by Jian Xin (Jae) Liao.
The modern round brilliant diamond is getting a “new old” look with the use of Japanese laser technology. Diamond’s high refractive index and adamantine luster make it ideal for faceting to produce dramatic scintillation, brilliance, and fire. One of the oldest and simplest cuts is the cabochon, with its smooth convex dome. A popular cut among colored stones, the cabochon is rarely if ever used in diamonds due to the gem’s extreme hardness and the difficulty in polishing it into a smooth, rounded surface. The New York lab recently examined several diamonds with an interesting variation on the cabochon.
 
About 30 buff-top round diamonds ranging from 0.25 to 1.50 ct, originally submitted to GIA’s Japanese lab, were received for grading. The buff-top cut has a low cabochon dome with a faceted pavilion (figure 1). These diamonds were cut with four short main pavilion facets (figure 2). This rare cut posed a challenge for the grading staff, as the smooth dome and faceted pavilion combined to produce internal reflections that made it very difficult to see clearly into the stone. Although diamonds can be cut with lasers, the polishing process with the use of diamond abrasives into a smooth rounded surface is both laborious and time consuming. The unfaceted dome was likely created using the same technology that produced the first synthetic nano-polycrystalline diamond sphere in Japan in 2011. One Japanese study (T. Okuchi et al., “Micromachining and surface processing of the super-hard nano-polycrystalline diamond by three types of pulsed lasers,” Applied Physics A: Materials Science & Processing, Vol. 96, No. 4, 2009, pp. 833–842) found that pulsed lasers most efficiently produce a smooth, undamaged surface for fine finishing diamonds. A combination of three lasers was used: a near-infrared laser for the rough shaping, and ultraviolet and femtosecond lasers for fine finishing (E. Skalwold, “Nano-polycrystalline diamond sphere: A gemologist’s perspective,” Summer 2012 G&G, pp. 128–131).

Lab Notes
Figure 2. Viewed face-up, the buff-top diamond shows an arrangement of four main pavilion facets. Photo by Jian Xin (Jae) Liao.
This advancement in diamond finishing offers new possibilities for a variety of interesting shapes and forms that could not be achieved with previous methods of polishing and faceting alone. Diamond cabochons, completely smooth spherical beads, and sugarloaf cuts could emerge next.