Ram's horn shell on copper
Spirula spirula is a type of cephalopod related to squid and cuttlefish. It is not closely related to Nautilus or extinct ammonites. The structure is an internal and stores gas and acts as a buoyancy compensation device.
Live Spirula live in deepwater in the tropics. When they die their shells float to the surface. Ocean currents distribute them widely. This one was found at Jervis Bay on southeastern coast of Australia.
This shell is lying on a piece of roughly beaten copper. Light is reflecting back from the copper to the shell.
For more information see en.wikipedia.org/wiki/Spirula