Most shells are calcite or aragonite, both of which are CaCO3.
Reefs are carbonates.
Many tropical islands exist as carbonate accumulations above volcanoes, and this includes Hawaii. The big difference between carbonate and siliciclastic sediment is that carbonate can form in quiet water sedimentary environments as shells and doesn't have to be transported into those environment. Also, reefs can grow far out from shore and provide a source of course sediment into deeper water environments than can be transported by marine processes. This can significantly change the character of sediments. However, except for the formation of shells and reefs, many of the physical transport features are similar for carbonates and other sediment types.