In 1964 the Yellow Oppenheimer Diamond Crystal was discovered at the Dutoitspan Mine in Kimberly, South Africa. The American celebrity jeweler Harry Winston later acquired the jewel from diamond merchant De Beers, and donated it to the Smithsonian Institute in honor of the late Sir Ernest Oppenheimer, a South African gold mining entrepreneur and chairman of De Beers. The 253.7-carat yellow diamond is nearly perfectly formed. It is one of the largest uncut diamonds in the world.

The Yellow Oppenheimer boasts an eight-sided double pyramid or octahedral shape, which is typical for diamond crystals. Though the stone is incredibly rare and unique, it cannot be regarded as a gem since it has not been cut. Only stones that have been cut and polished can be categorized as gems. It is for the same reason that the Yellow Oppenheimer does not have clarity, color, or cut grades. Nevertheless, it is believed that the famous diamond possesses an intense yellow color.

Measuring 3.8 centimeters in height, and approximately 20 by 20 millimeters, the yellow diamond received its rare color from impurities such as nitrogen, which substituted some of the carbon atoms as the diamond crystal formed.