In the daylight, this is an articulated Exogyra shell from the Cretaceous New Egypt Formation as it runs through Mullica Hill, New Jersey. I brought it home because it had an interesting bit of vivianite replacement covering half the surface of one valve. When I brought it home, I noticed some white material inside the cavity of the broken shell. I figured it might be calcite, which sometimes fluoresces. So, I pulled out my UV lamp. To my shock, not only did the white material glow an interesting powder blue color, but the majority of the one valve glows an intense, bright red! Meanwhile, the other valve doesn't glow at all.
Scroll right to see what it looked like in the dark with the UV lamp.