"In its purest state, a diamond is composed of carbon atoms and is colorless, like the De Beers Millennium Star," writes Jeffrey Post, curator of the National Gem Collection at the Smithsonian's Natural History Museum. "When certain foreign elements replace carbon atoms, they impart tints of color." Yellow diamonds get their color from nitrogen impurities. Blue diamonds get their color from boron. "Diamonds that are pink to red and orange in color have imperfections at the atomic level," notes Post. "Green, blue-green, and a small number of blue diamonds are a result of exposure to radiation over millions of years in the earth." When white light strikes the gem, certain color wavelengths are absorbed, while others are transmitted out. Those transmitted color wavelengths combine to create the color that we perceive.