Celeb jeweler Michael Greene, who designed Paris' multi-million dollar bling, tells TMZ ... Chris reached out to place his order 5 to 6 months ago, back in the summer.
Keep in mind ... Paris and Chris had just gone public with their relationship back in February, so it was likely less than a year into things that he decided he wanted to lock down Paris.
Greene also tells us the huge diamond -- 20 carats strong -- took about 4 months to cut, and the setting only took a couple days. He says Chris came in to get it right before their Aspen ski trip.
We're told Chris was shaking when he picked it up because he was "so nervous." With that kind of hardware in his pocket, we would be too.