This is a fascinating fossil colus/turritella agate cabochon. This one is an excellent collection of shells, agatized sections, as well as a couple of small druzy quartz sections. Very unusual.
Turritella is a genus of medium-sized sea snails. They have tightly coiled shells, whose overall shape is basically that of an elongated cone. These ones are naturally fossilized. The name Turritella comes from the Latin word turritus meaning "turreted" or "towered" and the diminutive suffix -ella.
Although this is a cab, you can also glue on "bails", or have the piece drilled to accept a pinch bail. Either approach will let you turn this sample into a fine pendant.