The 4.22 ct faceted celestine in Figure 1 is reportedly from Madagascar and was characterised by the author to add more gemmological data to the literature on this collector’s stone. It showed the following properties (Table 1). These characteristics are consistent with those previously reported for celestine (O’Donoghue 2006^[5]). Microscopic observation (up to 40×) showed no visible inclusions.

Table 1. Observational and measured properties

Infrared reflectance spectroscopy:

Infrared (IR) reflectance spectra were collected from several spots on the stone’s table and pavilion facets, and a representative spectrum is provided in Figure 2. Although the anisotropy of the material significantly modified the pattern in the 1300–1000 cm^–1 range, overall the data matched the reference spectrum published by Hainschwang and Notari (2008)^[6]. It is possible to distinguish celestine from baryte using IR spectroscopy because the ν₁ band (stretching mode) is at 991 cm^–1, versus 981 cm^–1 for baryte. Three bands (ν₄) at 614–618, 643 and 654 cm^–1 for celestine (compared to 610–614, 635 and 648 cm^–1 for baryte) can also be used, but the patterns of these bands are sensitive to anisotropy. The bands’ assignments were reported by Lane (2007)^[7].

UV-VIS-NIR spectroscopy:

Visible-range spectra (Figure 3) were collected for each polarisation direction corresponding to the three pleochroic colours, and the path lengths were used to calculate absorption coefficients. The E||b spectrum (green-blue) consisted of two broad bands at 425 and 618 nm that created a transmission window at about 510 nm. The E||c spectrum (bluish violet) was similar to E||b, but one of the broad bands (605 nm) was somewhat shifted towards the blue region and it lacked the 425 nm band. An asymmetric band (shoulder) on the low-wavelength side of the 605–618 nm absorption is due to an underlying band at around 575–585 nm. The E||a spectrum (colourless) was rather flat.

The causes of colour in celestine were studied with EPR spectroscopy by Bernstein (1979), who reported the presence of colour centres due to radiation damage; no more recent work appears to have been published. Bernstein (1979)^[1] indicated that the 425 nm band is related to SO₂^– , the 575–585 nm band might be due to SO₃^– and the 605–618 nm feature is associated with O^–. These centres are stabilised by the presence of trace components, primarily K⁺ substituting for Sr²⁺. Bernstein (1979)^[1] also performed heat-treatment experiments and found that thermal bleaching occurred at just 200°C for heating durations of a minute to several days; the colour could be restored by irradiation

Photoluminescence spectroscopy:

Celestine has been reported in the literature to be sometimes ‘fluorescent’, with a white, blue or even red reaction to UV radiation. A broad photoluminescence (PL) emission band usually peaking between 430 and 500 nm has been possibly connected to Pb²⁺ centres, but this has not been confirmed (Gaft et al. 2015^[8]). Other studies have proposed several other possible luminescence causes, such as organic impurities and Eu²⁺ (Tarashchan 1978^[9]), and at low temperature (77 K), O₂^–, VO₄, TiO₄ and MoO₄ (Gaft et al. 1985^[10]). The stone described here was analysed by PL spectroscopy using 254, 375 and 405 nm excitations at room temperature, but no photoluminescence was detected.