Kate wore this to the Order of the Garter Service in 2011.
Here is the closeup.
I have not been able to find the jewelry company that made this. Diamond cross pendants are not uncommon or unique, and many companies make very similar looking pendants.
I had an idea that Annoushka could have made this, but I don’t think I’m correct about this. Here is the pendant if you want to take a look. Kate’s pendant is stockier and less elongated (although it is not an equal-armed cross).
Kate’s pendant is made of diamonds set in white gold The top point and the side points of the cross appear to be of equal length. The bottom point is slightly longer than the others. The chain it hangs from appears to be made of white gold.
She also wore in during a trip to Santa Barbara, California in 2011.