Diamond Head got its English name from British sailors in the 19th century who mistook calcite crystals on a nearby beach for diamonds. Today a casual 1.1km (0.75 mile) hike leads to the edge of the crater with a gain of 560 feet in elevation.
At the top of Diamond Head you'll find a former coastal artillery observation platform, small bunkers, tunnels, an interior spiral staircase, and a long exterior outdoor staircase. We were pretty casual and took our time at Diamond Head, and the hike and sightseeing still only took a couple hours. It's well worth a visit for the views alone.