Aloha Sandra-
Its Hawaiian name is Le’ahi, and it is said that Hi’laka, the sister of the fire god Pele, named the crater Le’ahi because the summit resembles the forehead (lae) of the yellowfin tuna fish (‘ahi).
It wasn’t until the late 1700’s, when western traders thought they had found diamonds on the slope of the crater, that they began to call it Diamond Head. But before you head to the Diamond Head State Monument to dig for diamonds, you should know that (to the disappointment of the western explorers and traders) the so-called diamonds were merely calcite crystals, worthless to mine. However, as you already know, Diamond Head is a priceless jewel of nature in its own right.
Mahalo for your gem of a question,
Big Kahuna