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I was playing around with the UV lamp in my lab, seeing what might unexpectedly glow this afternoon. This was a nice surprise. It's an internal mold of Bacculites sp. with sutures that fluoresce orange under 345nm UV light. Bright orange like this usually indicates calcite, a mineral that makes up fossil shells and some modern ones, too. Between the mud-filled chambers, the shell was preserved while the exterior of the cone wore away. The shell material was either calcite to begin with or, more likely, began as aragonite (same chemical compound as calcite, but different crystal structure and glows yellow instead of orange) and changed over millions of years to the more stable configuration of calcite. Meanwhile, the mud looks like it may have a little bit of some fluorescent minerals in the mix, but it's mostly a daylight-only affair. The blue may be some residual glue from a label. This specimen is from the late Cretceaous Pierre Shale Formation in South Dakota.