This post shows some images of a Giant Venus Clam fossil (aka Mercenaria permagna) of the Fort Drum Member, Nashua Formation of Ruck's Pit in Okeechobee County, Florida, USA. This animal lived in the Calabrian Stage (780,000 years ago to 1.8 million years ago) of the Pleistocene Epoch of the Quaternary Period.
What is special about this fossil is that the calcite crystals that form inside the shell are yellow fluorescent to ultraviolet (UV) light. The above image was using an LED array to illuminate it.
Thanks to Rusty for this fossil.