Dark purple chrysoberyl

By Dr. M.S. Krzemnicki, first published in Facette 28 (May 2023)

Figure 1: Dark purple chromium-rich chrysoberyl. This gemstone does not show a colour change and thus cannot be called alexandrite. Photo: A. Chalain, SSEF.

Since about three years, we see at SSEF occasionally chrysoberyl samples of very dark purplish to purplish brown colour, sometimes of quite impressive size (>10 ct). Based on chemical composition, these chrysoberyls show distinct concentration of chromium, but no evident change of colour, thus do not fit the criteria to be called alexandrite (see e.g. alexandrite definition by LMHC, information sheet No. 9, https:// www.lmhc-gemmology.org/gemstones). In fact, these stones contain much too high concentrations of chromium – nearly 3 wt% Cr2O3 in the chrysoberyl of Figure 1 – to create a colour change effect in these stones when observed under different standard illumination (daylight and incandescent light). The high chromium concentration results in two very dominant chromium-related absorption bands at about 570 and 410 nm in the absorption spectrum (Figure 2). As a consequence, light transmission is mainly restricted to the reddish part of the visible range with some minor contribution by the “transmission window” in the blue- green range, hence the dark purple colour of this chrysoberyl. In addition, the high chromium concentration increases the general absorbance of the stone, thus deepening the colour with a dark tone.

Figure 2: Absorption spectrum of a Brazilian alexandrite (blue spectral curve) exhibiting a distinct colour change compared to the described chromium-rich dark purple chrysoberyl. The spectra have been vertically stacked for clarity. Figure: M.S. Krzemnicki, SSEF.
Dark reddish chrysoberyl containing high amounts of chromium have been reported before in literature, mainly from Brazil (Schmetzer et al. 2014, Stone-Sundberg 2014). This rather new material of partly important size is reportedly from East Africa, however the precise location is not known to us so far. To summarise, the above described properties (no colour change, only dark purplish to purplish brown colour) mean that such chromium- rich stones do not qualify to be called alexandrite and are identified as chrysoberyl on SSEF reports.