On blacklights
I believe we are talking about ultraviolet lamps.  Many mineral
collectors have displays of fluorescing minerals, and they use
ultraviolet lamps.  There are actually two lamps, a short-wave UV lamp
and a long wave UV lamp.  Many minerals fluoresce under long-wave UV,
and fewer fluoresce under short wave.  A mineral may fluoresce different
colours under the different lights.
I don't know which type shells need, but probably the former, especially
if the cause of the shells fluorescence is the presence of fluorite.
Where to get these lights?  Rock & Mineral or lapidary shops may have
them.
Hope this helps.
Nora Bryan
Calgary, AB
CANADA