Matching her new earrings closely, Kate also wore this necklace on the first day of the 2017 Poland/Germany tour.
The necklace’s pendant looks like an oval version of her tanzanite pendant necklace. Small white diamonds frame the oval ruby stone in the center. The pendant looks to be set in white gold, and the chain looks to be made of white gold.
There is some discussion about whether or not this is a G Collins and Sons piece. It certainly looks like it, but we would need to wait for confirmation. I don’t think this is a set to go with her earrings because the settings are completely different.
Here it is on Kate: