Housekishou Richard-shi no Nazo Kantei

On a seemingly ordinary night, university student Seigi Nakata rescues a beautiful Englishman named Richard from a group of drunkards. Richard is a jeweler who travels around the world to sell jewels to clients. Together, the two begin to solve mysteries surrounding the jewels they come across and their owners, starting with the mysterious pink sapphire ring that Seigi's grandmother left to him.

(Source: Shueisha, translated)