On May 24, 2011, one of Kate’s first engagements upon returning from her honeymoon was to meet with the Obamas.
She accessorized her lovely Reiss Shola dress with this shell necklace.
The necklace appears to be made of elongated, curved pale beige shells speckled with a black pattern. I don’t possess enough knowledge of conchology to identify what type of shells they are. I speculate that they could be some type of cowry shell. I found these and these images of cowrie shells. They both look similar to the pattern on Kate’s necklace.
Here it is on Kate.