Looked in several slang dictionaries as well as the reliable old OED. No luck.
The expression seems logical, however. Sounds like Joy was condensing to just the pertinent information, getting the highlights, or (many expressions) sorting the wheat from the chaff, looking for the jewel, highlighting, 'where's the beef? etc. etc. Looking for the kernels or the gems in a wad of bumff = gem down? Why not?

(I saw an expression in the newspaper this morning that was new to me, and used in a way not found in any dictionary, but I will post in a separate thread so that I will not dilute Mr. Red's in(en)quiry)