There's something going on here, but I can't reconcile the piece with maybe antler bits with the piece that does show some bilateral symmetry. Are there two objects in these images?
I'm thinking it may be the cast from the cavity inside a skull. Teh calcium of the skull eventually eroded away without fossilizing - leaving the "mold" to harden into rock. Maybe. But only if there are two objects in these images.
Natural, unusual stone. It's appearance has been enhanced with the use of a flat tipped tool ( screwdriver ). You can see the marks most clearly, shaping that dividing line.
Calcium based stones but maybe small fossel which is very degraded in your bottom photo from the same strata. How many inches or feet down was that last one dug?