Blue Diamond: The Rarest and Most Precious Diamonds come in different colors, including yellow, brown, green, pink, and red. But of all these hues, blue diamonds are perhaps the rarest and most precious. Blue diamonds are placed alongside purple and red as some of the rarest and highest valued natural colored diamonds. They are created when diamond crystals combine with boron. Blue diamonds encompass a range of tones, hues, and saturation levels, from faint powder to Rich Ocean and deep Sapphire-like blue.