First Appearance: Daring Mystery Comics #7 (April 1941).
Golden Age Appearances: Daring Mystery Comics #7-8.
Modern Apperances: Marvel Premiere #29-30, Invaders #6, #35-37, Marvel Two-in-One Annual #1, Marvel Two-in-One #79, Thunderbolts #46, Citizen V & the V-Battalion v2 #1.
Dates Active: 1941-present.
Professor Elton Morrow is exploring in the Antarctic when he finds an enormous diamond which glows with a blue light. He returns to the US via a ship, but when the ship enters the North Atlantic it gets torpedoed by a U-boat. Morrow grabs the box the diamond is in and uses it as a flotation device, but the U-boat sees him and machine guns him, blowing up the box and the diamond and embedding the fragments of the diamond into his body. This gives him a mostly-diamond body, which makes him bulletproof, mostly invulnerable, and superstrong. He uses these powers to fight crime and evil.
In the modern era he was shown in action during World War Two as a member of the Liberty Legion. Citizen V v2 #1 showed the Diamond, in costume, attending the funeral of Union Jack (II), which does make me speculate about how long he remained active as a hero. MTIO #79 showed the Diamond in retirement on Cape Cod, his powers never having faded but his having grown old nonetheless. In that story he was visited by the Star-Dancer and apotheosized by her, and left with her for outer space. In Quasar it is revealed that the Star-Dancer was captured by the alien the Stranger after she and the Diamond left Earth; what happened to the Diamond is not known. In Thunderbolts #46 it was revealed that the Blue Diamond's diamond was actually a sliver of a Lifestone, a power gem created to form an evolutionarily advanced group of the Guardians of the Galaxy; other figures who had slivers of the Lifestone included Man-Wolf, Moonstone, the Basilisk, Ulysses Bloodstone, Dr. Spectrum, and the Sphinx.
Note: Whether or not Blue Diamond or Diamondhead appeared in Quasar #16 is a subject numerous e-mail correspondents and I have debated, with (I'm sorry to say) many of you arguing the Diamondhead case with a lack of grace and tact. (Fer cryin' out loud, people, I've worked hard on this site. The least you could do, before telling me I'm wrong about something, is throw me a few kind words. Starting out e-mails with the words "You're wrong about" is not a way to get into my good graces.)