Diamond Head is the name of a volcanic tuff cone on the Hawaiian island of Oʻahu and known to Hawaiians as Lēʻahi, most likely from lae ‘browridge, promontory’ plus ʻahi ‘tuna’ because the shape of the ridgeline resembles the shape of a tuna’s dorsal fin. Its English name was given by British sailors in the 19th century, who mistook calcite crystals on the adjacent beach for diamonds.
Diamond Head is part of the system of cones, vents, and their associated eruption flows that are collectively known to geologists as the Honolulu Volcanic Series, eruptions from the Koʻolau Volcano that took place long after the volcano formed and had gone dormant. The Honolulu Volcanic Series is a series of volcanic eruption events that created many of Oʻahu’s well-known landmarks, including Punchbowl Crater, Hanauma Bay, Koko Head, and Mānana Island in addition to Diamond Head.
Diamond Head, like the rest of the Honolulu Volcanic Series, is much younger than the main mass of the Koʻolau Mountain Range.
The eruption that built up Diamond Head was probably very brief, lasting no more than a few days. It was probably explosive, since when the cinder cone was originally formed, the sea level is thought to have been higher and the vent burst erupted over a coral reef. Another factor probably contributing to the eruption’s explosive nature was that rising magma would have come into contact with the water table. The eruption’s relatively brief length is thought to explain why the cone today is so symmetrical.
A nearby eruption that took place at about the same time as the Diamond Head eruption was the eruption that built the Black Point lava shield. Since the type of eruptions that built Diamond Head tend to be monogenetic, geologists don’t believe that Diamond Head will erupt again.
In 1968, Diamond Head was declared a National Natural Landmark. The crater, also called Diamond Head Lookout was used as a strategic military lookout in the early 1900s. Spanning over 475 acres (including the crater’s interior and outer slopes), it served as an effective defensive lookout because it provides panoramic views of Waikīkī and the south shore of Oahu.
The Diamond Head Lighthouse, a navigational lighthouse built in 1917 is directly adjacent to the crater’s slopes. In addition, a few pillboxes are located on Diamond Head’s summit.