Despite all the advertisements for engagement rings and necklace gifts, natural diamonds are imperfect. Their carbon atoms are arranged in a cubic lattice, but they nonetheless contain several types of crystallographic defects due to impurities that occasionally replace a carbon atom. The most common types of impurities are nitrogen and boron; "Type I" diamonds contain nitrogen impurities, which can be isolated or clustered, at concentrations of up to 1%, and make up about 95% of all natural diamonds.