Let me see if I've got this fairly straight
.the Jarvis Diamond is "secretly" being sent down to a retired jeweler living in a little town named San Juan, there to be broken up into small pieces for safe disposal. A man named Morgan rolls into town and checks in to a boarding house to lay in wait to steal the diamond, posing in the meantime as a professor researching a book on a historical figure from the area. Our hero (Kane Richmond) checks in to the same boarding house and announces that he himself is in town to—you guessed it—research a book he's writing on the same figure. Meanwhile, several of Morgan's henchman have arrived in town under separate cover: they are allegedly training a featherweight fighter for a bout that may or may not be coming up in the foreseeable future. This "boxer" turns out to be our other hero, Frankie Darro.
Richmond strikes up a romance with the landlady, June Gale (who is the daughter of the retired jeweler), while young Darro finds himself the object of attentions of Rosita Butler, an eager young lady who spends the entire picture chasing after Frankie and being rebuffed.
Yes, that's about it. Darro picks a lot of fights with the dull-witted henchmen. Richmond kind of hangs around waiting for something to happen. Gale dotes on her elderly father and tries to get him to lay off of working so hard on cracking up this diamond. Butler eventually steals a peck on the cheek from Frankie Darro. The bad guys grumble about having nothing to do.
Not a lot of twists in this plot. And I've got to say that this film contains more than the usual number of moments where a character does something really dumb. For example, if you want to hide a teabag containing diamonds, don't fold it up in this morning's newspaper sitting right on the kitchen table! Duh! However, The Devil Diamond has got some decent action and some energetic performances—at least the cast look like they're trying. And so it's obviously worth a look for us fans of the "comedy-mystery B movie" genre.