Table 1. Observational and measured properties. These data are consistent with those reported for enstatite by O’Donoghue (2006^[7]).

Light scattering from a dense network of inclusions formed the six-rayed star, with the strongest rays seen parallel to the length of the cabochon. Microscopic examination (figure 2) revealed parallel acicular (needle-like) inclusions (up to 2 µm wide and 150 µm  long) aligned in three directions and intersecting at approximately 57° and 62°. The intersection angles are consistent with the orthorhombic pseudo-hexagonal symmetry of enstatite and published data: 57° and 64° (Eppler 1967^[1], 1971^[2]); 57° and 61° (Henn & Bank 1991^[3]); and 64° (Schmitz et al. 2016^[4]). Eppler (1967^[1]) suggested rutile as a possible inclusion; Henn & Bank (1991^[3]) ascribed the asterism to hollow channels; Gübelin & Koivula (2008^[8]) indicated rutile or sillimanite in Sri Lankan enstatite; and Schmitz et al. (2016^[4])  identified ilmenite in Norwegian enstatite. Further investigation such as micro-Raman spectroscopy would be necessary to identify the inclusions in the Madagascar enstatite.

Infrared reflectance spectroscopy:

Infrared reflectance spectra were collected from several spots on the cabochon, and all yielded the same pattern without any distinct difference (figure 3). The spectrum is characteristic of enstatite (see the Enstatite-Ferrosilite database; Chihara et al. 2002^[9]).

UV-VIS-NIR spectroscopy:

Vis-NIR spectra (figure 4) were collected with the beam entering from the top and oriented nearly perpendicular to the base of the cabochon. Due to the beam’s orientation, only two spectra were acquired: one for the orangey brown colour and one for the greyish green colour. The spectra were consistent with reference spectra for enstatite (see, e.g., the Enstatite-Ferrosilite database; Caltech^[10]).

Across the visible range, both spectra showed a continuous absorption increasing towards the UV region (weaker for the greyish green direction), with a superimposed series of narrow bands at 427, 446, 482, 506 and 549 nm; the 506 nm band is characteristic of enstatite.In the near-IR range, both spectra show a broad, strong absorption band centred at 910 nm (stronger in the orangey brown direction). The continuous absorption in the visible range coupled with the near-IR band creates a transmission window in the orange and red region, which is narrower in the orangey brown spectrum than in the greyish green one. The narrower transmission window produced the orangey brown colouration, while the broader one resulted in the greyish green colouration since more green was transmitted. The continuum towards the UV and the strong band in the NIR are related to Fe²⁺ substituting for Mg²⁺ in the M1/M2 sites, and the narrow bands between 400 and 600 nm are Fe²⁺ spin-forbidden absorption bands (Hazen et al. 1978^[11]; Zhao et al. 1986^[12]). Thus, the stone is coloured by Fe²⁺.

Enstatite (also referred as orthoenstatite, the orthorhombic form) has few polymorphs among which clinoenstatite is the monoclinic form. The clinoenstatite's RI, birefringence, optic sign and SG are similar in all respects to those of the enstatite (Deer et al. 1997, pp. 118–119^[6]), however the characteristic pale-pink to green pleochroism distinguishes many (ortho-)pyroxenes from clino-pyroxenes. The strong orangey brown to greyish green of this sample is consistent even if not exactly similar to that pleochroism, it points enstatite out.

The enstatite polymorphism has been studied with IR and Raman spectroscopy, Chihara et al. (2002)^[9] indicated that far-infrared (~400–10 cm⁻¹) spectral features provide the best way to distinguish them, however it is also possible to use the mid-infrared range (~4000–400 cm⁻¹). The (ortho-)enstatite and the clinoenstatite mid-IR spectra from that study are provided in figure 5 for comparison to the sample's one. They were numerised and the frequency scale converted to a linear one, therefore they show some slight discrepencies with the sample in the bands position that are slightly shifted. The discriminating features pertain to the variation of the bands at 725, 694, 484, 455, 434, 422 and 405 cm^–1 shown in the pink highlighted regions of figure 5. This sample of enstatite from Madagascar shows an IR spectrum which is closer to the enstatite's one than to clinoenstatite's one.