En medio del coral
This is a worn valve of a foam oyster. Gryphaeidae are "living fossils", a very interesting small family with only a few surviving species.
When the shells wash up on Nevis, St. Kitts, and St. Eustatius, the valves are usually broken and worn. In fact, most of the time you only find very small worn pieces that seem barely recognizable. But the tiny foam-like structure of the upper shell layer is very distinctive, and makes the shell easy to ID.
The broken pieces are sometimes confused with pieces of Denostrea frons, a true oyster, which also has a shell with a purple exterior. However, valves of that species are much thinner, have no foamy structure, and their interior usually has a metallic-looking appearance.
This is a ringer!
I considered listing this as "human", because these northern oysters were imported over a great distance to Nevis, consumed as a meal, and then the shells thrown away.
This shell ended up on the beach with the beach-drift local shells.
This kind of thing can be very confusing to a beginner!
This is the second Crassadoma gigantea (Giant Rock Scallop) I've found in two days, and third in five years!