On March 17, 2017, Kate wore this mabe pearl necklace to a reception held at the British Embassy in Paris.
Many of us suspected it could be the matching necklace to the Balenciaga earrings she also wore. If one looks at the border of the pearl setting, it lacks the crystals the Balenciaga earrings feature.
One of the more unique pieces Kate owns, this necklace features four large mabe pearls suspended in a somewhat asymmetrical pattern. One mabe pearl is centered on a metal chain (possibly white gold) and under this are two pearls on slim chains, the left chain considerably shorter than the right chain. The second pearl on the right side has one more pearl attached below it. The chain drop is shorter than the drop for the pearl above it. This gives it the aforementioned asymmetrical effect.
Here it is on Kate: