Largely tropical mollusc with a glossy patterned shell (6)
I believe the answer is:
cowrie
(Other definitions for cowrie that I've seen before include "Shell once used as currency" , "Marine gastropod with a shiny shell once used as money in some parts" , "Shell sometimes used as form of money" , "Brightly marked seashell" , "Mollusc (shell used as money)" .)