Brow of the ʻahi
Lae is a brow or point and ʻahi is the yellowfin tuna. Seen from the west the ridgeline has the profile of a tuna's dorsal fin, and the name records that.
Another account credits the name to Hiʻiaka, Pele's sister, who is said to have remarked on the resemblance.
Why Diamond Head
British sailors in the 1820s found calcite crystals in the crater rock and took them for diamonds. They were not. The English name stuck anyway.
The crater is a tuff cone about 300,000 years old, formed in a single explosive eruption. It was fortified by the US military from 1904 and is now a state monument and the most-walked trail in Hawaiʻi.