Octahedra crystals are not polished enough to read the RI with the gemological refractometer but the IR reflectivity meter gives a  RI about 1.79, the Specific Gravity of each crystal is between 4.59 and 4.61, all stones together yield an average of 4.60. Both RI and SG are inconsistent with the spinel specie. Spinel belongs to the spinel isostructural subgroup (oxyspinel group, spinel supergroup according to IMA 2018) where 28 species are identified and only 6 with predominant Al with the formula (Y-Al₂O₄). The most commons ones are the spinel (commonly known in gemology) with Y=Mg, the hercynite with Y=Fe, the galaxite with Y=Mn and the gahnite with Y=Zn. Series exist between the corresponding end-members, and intermediate members have been found.

Table 1. Observational and measured properties

Infrared reflectance spectroscopy:

The infrared reflectance spectrum (figure 2) was collected from a face of the 2.02 ct crystal, spectra were also collected from other crystals but all yielded the same spectrum pattern. The spectrum's pattern is consistent with oxydes' spectra and shows some similarities to the spinel ones (see spinel database) but the 400-600 cm^-1 range shows significant differences with two distinctive bands at 500 and 565 cm^-1 which characterizes the crystal as gahnite, the Zn isostructural spinel (see gahnite database, Green gahnite from Brazil and Chukanov's book^[3]).

UV-VIS-NIR spectroscopy:

The Vis spectrum (figure 3) was collected from the 2.02 ct crystal, the light path going through the crystal between two opposite and parallel faces. The absorption band wavelengths were calculated by gaussian fitting. The spectrum's pattern consists in a transmission window in the blue delimited by an edge below 400 nm and a wide group of bands between 530 and 650 nm. In the middle of the transmission window, there are two additionnal bands at 460 and 477 nm, respectively ascribed to Fe³⁺ and  Fe²⁺. The edge in the violet is likely caused by an Fe²⁺ band superimposed to a wing of a large O^2--Fe^2/3+ LMCT band in the ultra-violet. Between 530 and 650 nm, the bands at 545, 580 and 626 nm are ascribed to d-d spin-allowed transitions ⁴A₂(F) → ⁴T₁(P) of tetrahedrally coordinated Co²⁺, and that at 559, 574 and 598 nm are ascribed to spin-forbidden transitions ⁵E → ³T₂ of tetrahedrally coordinated Fe²⁺.  The band at 656 nm and 731 nm are ascribed to mechanisms involving Fe^2+/3+ pairs (IVCT and ECP). There is another wide band at 925 nm which is not displayed in figure 3 that is ascribed to the spin-allowed transitions ⁵T_2g → ⁵E_g of Fe²⁺. 

Bands' assignment in spinel has been studied for years, cobalt spectra^[4],[5],[6] are well known by gemologist even using a handheld spectroscope and are easily observable with Verneuil sky-blue spinel, however spectra become more complex as soon as iron and even manganese and chromium interplay. Works on the subject were published by Taran et al. 2009^[7], D'Ippolito et al. 2013^[8], 2015^[9] and were used for Fe²⁺ assignments.

This spectrum's pattern is well-known amongst the blue spinels colored by cobalt and iron (spinel database).

The on-going topic with blue spinels is to know if such spinel is a cobalt-spinel in comparison to the Luc-Yen spinel ?
During 2018, Palke et Sun (GIA) stated that cobalt, even at low concentration, gives the blue color to spinel although iron which is usually more abundant, and according to its concentration, modifies the darkness of the stone or makes it grayish. Therefore, a cobalt-spinel, is a spinel colored by cobalt with a vivid blue color, darker spinels cannot be qualified as cobalt spinel.  

The gahnite from Nigeria is mainly colored by cobalt but because of iron, its color does not qualify for cobalt-spinel color even if attractive.

Photoluminescence spectroscopy:

The photoluminescence spectra (figure 4) of the 2.02 ct octahedral crystal were collected for three excitation sources: 405, 444 and 532 nm. The 405 nm laser excited a weak luminescence at 647 and 737 nm, and none around 690 nm as it could be expected for Cr³⁺ in the red as for many spinels. The 444 nm laser excited a weak green luminescence at 512 nm and two very weak band at 640 and 737 nm. The 532 nm laser excited a moderate orange-red luminescence peaking at 650 nm.
The emissions at 512 and 737 nm are connected to Mn^{2+ [11]}. The emission at 650 nm while excited at 532 nm is ascribed to Co²⁺, the excitation falls in the vicinity of the ⁴T₁(P). With the 405 and 444 nm excitations, the emissions are respectively at 647 and 640 nm but comparatively much weaker than that for 532 nm excitation, they are possibly connected to Co²⁺ as well. 

The observed luminescences are similar to that of cobalt-bearing blue spinel (see spinel database).

Conclusion:

The IR reflectance spectroscopy identifies the octahedral crystals as gahnite the Zn isostructural spinel. The blue color mainly results of Co²⁺ and secondarily of Fe²⁺/Fe³⁺ but as iron absorbs a part of the blue at 460 and 477 nm, it likely causes the 'dark' blue color. The dark-blue color cannot qualify for the cobalt spinel color which is a bright, vivid blue color. The photoluminescence spectroscopy confirms the presence of Co²⁺ and Mn²⁺ but Cr³⁺ is not 'detected'.

^[1] Gem quality gahnite from Nigeria, B. Jackson, Journal of Gemmology, 1982, Vol. 18, No.4, pp. 265–276

^[2] Blue Gahnite from Nigeria, E. Boehm, B.M. Laurs, Journal of Gemmology, 2018, Vol. 36, No.2, pp. 96-97

^[3] Infrared spectra of mineral species, Nikita V. Chukanov, 2014, Springer Editor, ISBN: 978-94-007-7128-4

^[4] 'Cobalt-blue' Gem Spinels, J.E. Shigley, C.M. Stockton, Gems & Gemology, 1984, Spring, pp. 34-41

^[5] Cobalt colored Color-Change Spinel, P. Maddisson, Gems & Gemology, 1990, Fall, pp. 226-227

^[6] Cobalt-colored synthetic spinel, GRC, Gems & Gemology, 1991, Summer, pp. 113-114

^[7] Optical spectroscopic study of tetrahedrally coordinated Co²⁺ in natural spinel and staurolite at different temperatures and pressures, M.N. Taran, M. Koch-Müller, A. Feenstra, American Mineralogist, 2009, volume 94, pp. 1647-1652

^[8] Crystallographic and spectroscopic characterization of a natural Zn-rich spinel approaching the endmember gahnite (ZnAl2O4) composition , V. D'Ippolito et al., Mineralogical Magazine, 2013, Vol. 77, pp. 2941-2953

^[9] Color mechanisms in spinel: cobalt and iron interplay for the blue color, V. D'Ippolito et al., Phys Chem Minerals, 2015, Vol. 42, pp. 431-439

^[10] What Is Cobalt Spinel? Unraveling the Causes of Color in Blue Spinels, A.C. Palke, Z. Sun, Gems & Gemology, 2018, Fall, p. 262

^[11] Luminescent Spectra of Minerals, Boris S. Gorobets and Alexandre A. Rogojine, Moscow, 2002, ISBN: 5901837053, p. 175

^[12] Blue gahnite from Nigeria, M. Hain, Z. Sun, Gems & Gemology, 2019, Fall, pp. 434-436

^[13] Blue Gahnospinel crystals from Nigeria, J.M. Arlabosse, Gemmology Today, 2019, December, pp. 5-8