The Gates are said to be composed of "glittering, crystalline facets", just like the Crystal Arch itself.
While Hell's forces have fought their way to the Gates more than once, no account exists of the Diamond Gates having ever been breached.
In the final act of Diablo III, Diablo, now the Prime Evil, appears before Imperius and the Diamond Gates. Imperius arms himself with heavenly spear Solarian and confronts Diablo. The archangel breaks his vessel, Leah, and fights the demon. Diablo retaliates and stabs Imperius in the chest, wounding him. The Lord of Terror then proceeds to smash the Diamond Gates to the ground, opening the High Heavens for attack for the first time in the history of the universe.