IMDb Trivia: One of the "75 Most Wanted" films listed by the British Film Institute as "Missing, believed lost".
I saw this film on Netflix, so apparently it is NOT lost--at least not any more.
Dennis O'Keefe plays a Treasury Department agent who has been sent to Britain. This is because there may be an organization that has learned to produce perfect diamonds--ones that are so perfect it could destroy the market. The trail leads to a scientist--a scientist who has disappeared. Although the chances are ridiculous, the wanted man's daughter just happens to be a lady that the Treasury man was dating! He's afraid to tell her about everyone's suspicions about her father and he and Scotland Yard are baffled.
This is a decent British version of film noir. While it's not quite as dark as many films in the genre, it is tough and violent and enjoyable. However, it does start off very slowly--so keep watching and don't give up on this one--it's worth it.
If you do watch a couple things you should note. First, at one point in the film they talk about a remarkable fake diamond being produced in 1924. While it was believed in 1954 that this was true, apparently this was a hoax and that part isn't mentioned in the film. Second, when O'Keefe is chased by a truck he's knocked onto the cobblestones. Watch closely--these cobblestones bounce--revealing they are clearly props.