Diamond Head on Oʻahu is a volcanic tuff cone rising above Honolulu and one of the south shore’s defining landmarks. Known in Hawaiian as Lēʻahi, it took its English name in 1825 after British sailors mistook sparkling calcite for diamonds. Formed by a later eruptive phase of the long-dormant Koʻolau Volcano, it is far younger than the main Koʻolau Range and belongs to the Honolulu Volcanic Series that also created Punchbowl Crater, Hanauma Bay, and Koko Head. Its crater later became Fort Ruger, an early United States military reservation, and today it remains both a protected natural site and a powerful visual symbol of Honolulu.