solved! Fossil Identification.
ETA: Huge thanks to my knowledgable circle! luzula suggest "orthoceratite" and that's close enough for me.
I'd love to know what to call this slightly chatoyant 10-element spear-shaped fossil. It's 35mm long, 10mm at widest point narrowing down to 1mm. Embedded in black rock, shaped and polished and deployed as a pendant.
( it's in here )