PT Markings on White Metal - Is this Ring an Antique?

by Janet
(Orlando, Florida)

I have this engagement ring which was handed down to me. I don't know much about it but it looks antique.

The markings on the inside I think say the following:

PT18KFDC this is on the left inside of the ring

I think on the right it says:
3HD 057

I am trying to decide if it's worth selling it for the gold or trying to sell the entire ring.

I think it's a little less than 1/2 karat round diamond in the middle, it has a total of 8 small rectangular chip diamonds, 4 on each side and 4 round diamonds, 2 on each side.

There is a lot of engraving detail on all sides of the ring, and it's squared off on the bottom.