gems, sapphirine, Bo Rai, Thailand

Steve Illenberger describes the identification of sapphirine among rough rubies from Bo Rai, Thailand, following the recovery of several apparently black mineral fragments in November 1986. Although such material would normally have been discarded as worthless, closer examination showed that the specimens were not opaque but intensely dark green. One stone was cut and polished and submitted to the Asian Institute of Gemological Sciences for identification. Preliminary testing yielded refractive indices of 1.710–1.715 and a birefringence of 0.005. Its specific gravity was estimated at approximately 3.50 using methylene iodide. The specimen was inert under both long-wave and short-wave ultraviolet radiation and showed no reaction through the Chelsea filter. Spectroscopic examination was inconclusive. The most conspicuous diagnostic observation was strong pleochroism, with bluish green and yellowish brown colors so distinct that the change could be seen simply by rotating the stone in ordinary light. Attempts to obtain an interference figure were unsuccessful, although interference colors appeared between crossed polarizers. Microscopic examination was restricted by the material’s deep color; the cut specimen appeared relatively clean internally, apart from one long needle of undetermined identity. Another piece of rough displayed several six-sided surface etch features. An initial hardness estimate of 8–8.5 was also recorded. Because the available tests did not establish the identification conclusively, the material was submitted for X-ray diffraction analysis at the Gemological Institute of America’s laboratory in Santa Monica, which identified it as sapphirine. Illenberger summarizes the mineral as a monoclinic, biaxial silicate containing magnesium, iron, and aluminum, and compares his observations with published reference properties. These include refractive-index ranges of 1.701–1.729 for α, 1.703–1.732 for β, and 1.705–1.734 for γ; birefringence of 0.004–0.007; specific gravity of 3.40–3.58; and hardness of 7.5. Thus, the measured optical properties and estimated density fall within the cited ranges, while the preliminary hardness estimate exceeds the reference value. The article also notes a contemporaneous report by J.I. Koivula and C.W. Fryer of dark blue sapphirine occurring as an inclusion in ruby from Bo Rai. The account documents sapphirine’s presence in material associated with the locality’s ruby production and illustrates the importance of investigating unfamiliar minerals recovered during rough sorting.