18k gold necklace mark question

[925John]

This is a mark on an 18k gold 22" necklace.
The claw clasp is marked 18k
The end loops are marked with F and what looks like 13 kt.

The links are 5mm, and the weight of the chain is 50g.

Wondering about the diamond mark with 13 .

Help is much appreciated.

Thank YOU.

[silvermakersmarks]

That is an Italian mark. It takes the form of a number, for the maker, and 2 letters for the province. Without knowing the letters the maker cannot be identified. A clearer, in-focus picture is required.

Phil