If you watched The Fake (like I did), The Diamond Wizard (1954) is more of the same: Once more, Dennis O'Keefe is an America in England investigating a forgery (man-made diamonds here), once more he gets involved with the daughter of an old duffer who is the main suspect, once more things get very strange.
One of the strange things is the crime at the heart of the film: Selling fake diamonds, indistinguishable from natural except for their super-high quality. But these aren't money or artworks - perfect diamonds aren't worth any less because they are man-made, are they? I guess we'll have to take that as given.
Another oddity is that the old duffer - mad scientist, really - has his laboratory in a Frankenstein castle. Well, it's in England, I guess they have a lot of those around. Also, the villain is a giant "brute of a savage beast" who tends bar in a pub and raises camellias. It takes all kinds, doesn't it?
There's some noir atmosphere, a nicely done police procedural car chase, some light banter between O'Keefe, his English counterpart (Philip Friend) and the mad scientist's beautiful daughter (Margaret Sheridan). Mostly it's just batty fun. Good luck following the whole 83 minutes.