50 Cent -- Jeweler Says Rapper's Crew Brutalized Him ... Stole Expensive Jewelry

50 Cent is smack in the middle of a criminal investigation after a guy claims 50's goons pistol whipped him and stole a quarter of a million dollars in bling.

Robert Marin, who owns LAX Jewelry Co., tells TMZ he went to Vegas during fight weekend because he wanted to sell the rapper a watch and chain worth $250K. Marin says he met 50 at Drai's nightclub, showed him the jewelry and 50 was in, but they needed to negotiate a price.

Marin says they stayed at Drai's until the wee hours Saturday and 50's crew invited him to see Floyd Mayweather's gym ... an offer he couldn't refuse. Marin claimed when they arrived things turned ugly. He says 2 of 50's guys pistol whipped him, stole the jewelry and fled.

Marin has filed a robbery report and Vegas cops are investigating.

50's people say the story is BS ... his attorney Scott Leemon tells us, “On behalf of 50, I can adamantly deny that he or his security had anything to do with these baseless and false allegations.”