Jeweler to the Stars Lorraine Schwartz -- Janitor Cleaned Up with My $250K Diamond

Jeweler to the stars Lorraine Schwartz lost her $250,000 ring on the Golden Globes red carpet ... and the janitor who cleaned that carpet is now the target of a criminal investigation.

Schwartz -- who designed engagement rings for Beyonce, Kim Kardashian, Blake Lively and many others -- was wearing a 10.62 carat yellow diamond during the Globe festivities. Schwartz told cops someone might have surreptitiously slipped it off her finger.

Schwartz was wrong. The ring just slipped off her finger and according to legal docs an opportunistic carpet cleaner -- Henry Bernal -- scooped it up and brought it home to his girlfriend.

Law enforcement sources tell TMZ ... Bernal and GF Christina Gonzalez took the ring to Kevin Jewelers for an appraisal, and when the jeweler said the store wanted to buy it Bernal got greedy and took it to GIA (Gemological Institute of America) to get it certified.

GIA knew something was up because the ring already had a certification number, so they contacted cops.

Police interviewed Bernal, who said he never went to cops and told them about his find because he didn't trust them.

The FBI, LAPD and Bev Hills PD are all investigating.