Vibrant colors of a Spinel stone from Cambodia.
With the aid of a Motic microscope with fluorescence attachment, using a DAPI lp filter set, the available Spinel stones fluoresced with a weak red light under UV light, at an excitation wavelength of 375 nm.
The fluorescence of Spinel was studied as early as 1887 by De Boisbaudran who found a red glow in most specimens and some green fluorescence in a few. Crookes plotted in 1887 the red fluorescence spectra of Spinel and pointed out the particular sharp band at about 690 nm.
The general chemical formula of Spinel is MgAl2O4. It belongs to the oxides and hydroxides family. The most common fluorescence activators for Spinel are Cr3+ and Mn2+. Peaks in the spectrum are:
Cr3+ : lines at 676 (675), 686 (685), 698 (697), 708 (707), 712, 718 (717) nm
Mn2+ : broad band peaking at 550-612 nm
Due to the rough shape of the available small and rough Spinel stones, it was necessary to take a large series of photos and stack them in order to visualize the surface structure. The stones were too big in size to put them under an upright fluorescence microscope. However, the Motic inverted microscope AE31E came in handy here. Although fluorescence observed with the naked eye was very weak, the Moticam S12 coped well with the low light conditions.
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