Hi thank you for your question. PT is the abbreviation for PLATINUM. It is a white metal with a light greyish cast. Popular in the early 1900s. Around 1915 18KT WHITE GOLD was introduced , a prefect imitation of the relatively more expensive Platinum Jewelry.
18ct is the gold purity mark. Most likely the diamond ring is comprised of 18 carat gold and Platinum. Platinum is often used around the diamond setting to make the diamond appear larger and because Platinum is a such a strong metal it results in a very strong, robust, luxurious setting around a girl's best friend.
I'm sorry but unfortunately I cannot provide any info at all concerning the maker's mark from your description. There are many sponsor's marks similar to what you have described. A photo would definitely help. You are welcome to upload a photo in a follow-up submission.
