At the “Tenacity on the Tasman” premiere, Kate wore this simple necklace.
It appears to be a station necklace-type with alternating black and white discs along the length of the necklace. The discs look flattened and to be roughly the size of a U.S nickel.
I’ll keep an eye out for more photos of her wearing this piece. I am sure it is costume jewelry, although I can’t think of a company I have seen which makes a necklace similar to this.