This is perfectly circular in cross section. I am fairly sure this is a Tentaculitoid, but I've never really identified one before. These are somewhat rare where I fossil hunt. I've seen a few bits and pieces stuffed inside of gastropod shells. This one was fairly long, with at least another 1 cm of material broken off when I cut it out of the parent rock.
It is mostly white in color, but that is more shell preservation at this site rather than an important attribute. Most of the Tentaculitoids in books I've seen have ribbed ornament, whereas this has lengthwise ribs.
CG-0616—Unknown
Scale bar = 5 mm.