PD on a ring commonly represents palladium. Your ring might be palladium-plated if it's got PD published above it. HK is really a trademark that means Harriet Kelsall. They layout and supply jewellery all over the British isles and also to other places likewise.  The Sun refers to the hallmark https://edwinynxhr.estate-blog.com/36063787/considerations-to-know-about-jp88