The material you are discussing is routinely called "turritella marble"—and is a fossil material that is composed from very ancient turritella shells and fragments, now in a mineral base. This is not an uncommon stone. And it can be exploited from such large pieces that table tops can be made from it. (See the photo here, harvested from a Google search.)

Also beads (!).

However, the subject of this dialogue is the use of currently living turritellas, whose shells have been converted into ornaments. And, in particular, the columella has been isolated and made into pendants.

Jamey