This gypsum is waterclear but contains some humic acids. This organic stuff is accumulated more rapidly in the faster growing direction than in the other ones. It thus forms an hourglass-shaped fluorescence. The fluorescence originates in the pi-bonds in the humic acids.
Photo in SW-UV and white light.
Locality: Le Forest, Dept. Seine-et-Marne, France.
Source: Fluorescent Minerals Workgroup MKA.
