“Predominantly” red color diamonds, which are stones with stone with red as the primary color, with no secondary hues, are among the rarest items found in nature. They are so uncommon, that the Gemological Institute of America records show that over a 30-year period from 1957 to 1987 there was no mention of a GIA lab report issued for a diamond with “red” as the only descriptive term. Certain GIA researchers have surmised that the red color may partially be the result of defects in the atomic structure, possibly resulting from “gliding,” which refers to the slight movement of atoms along the octahedral direction of the crystal. Natural Colored Red Diamonds the Rarest of Gemstones.