Gemological and Spectral Characteristics of Gem-Quality Blue Gahnite from Nigeria

Table 6

UV-VIS spectral absorption bands’ comparison of the blue spinel.

Species	Absorption bands	Source
Gahnite	444 nm, 489 nm: d-d electron leap of ^TFe^2+ (^5E ⟶ ^5T2)	This work
	609 nm: spin-allowed transitions of ^TCo^2+ (^4A2 ⟶ ^4 T1)
Natural blue spinel (Fe, Zn chromogenic)	458 nm strong absorbent band: characteristic absorption band of Fe	[24, 25]
	555 nm: related to Fe^2+
Natural blue spinel (Co chromogenic)	300∼330 nm: the d electron transition of Fe^3+ (^6A1g ⟶ ^4A1g)	[24, 25]
	440∼430 nm: the outer electron transition of V^3+
	500∼650 nm: areas of concentration of strong and pronounced peaks
	595 nm: spin-forbidden transitions of ^TFe^2+ (^5E ⟶ ^3T1)
	550, 585, 625 nm: spin-allowed transitions of ^TCo^2+ (^4A2 ⟶ ^4T1)