Turritella

Turritella was an agate made up of many small fossilized shells.

Description
The shells were spiral and cone-shaped, less than an inch (two and a half centimeters) long. The rock substrate was a dark brown. It was quarried in slabs and could be used instead of marble for facings or it was cut for inlay work. A typical price was 8 sp per pound (0.45kg).