Diamond Head was built by hydro-magmatic explosions that ripped through 200,000 year old coral reefs and Ko‘olau basalt. As a result, large pieces of coral (you can see the coral chunks here) and basalt are mixed in the tuff and magmatic debris of the cone.

The eruption most likely occurred in a very short period of time from days to perhaps a month, as suggested by the symmetry of the cone (Macdonald et al. 1986).