Johnny Depp Cuts Off Finger in Fit of Rage ... Amber Claims

Johnny Depp accidentally cut off his fingertip in a fit of rage ... then dipped the stump in dark blue paint and wrote on the wall in paint and blood the name "Billy Bob" -- the actor with whom Johnny accused Amber Heard of cheating -- this according to Amber and her people.

TMZ has obtained this gruesome photo of Johnny's butchered finger -- WARNING ... EXTREMELY DIFFICULT TO LOOK AT. It's an exhibit in her domestic violence case against Johnny.

We're told the incident went down in March of last year, just a month after Amber and Johnny married. They were in a villa in Australia when a fierce argument erupted.

According to Amber's version of events Johnny, who was drunk and high on ecstasy, smashed several bottles and windows and also slammed a plastic phone against the wall unit. She claims a piece of one of the items cut off a portion of the middle finger on his right hand.

Amber's version goes on ... she's a painter and had dark blue paint in the room. She claims Johnny dipped what remained of his forefinger in the paint and then used the gnarly digit to write various words on a mirror, including "Billy Bob" and "Easy Amber."

Amber had done a movie with Billy Bob Thornton and she claims Johnny accused the 2 of sleeping together -- something she strongly denied, adding he accused her of infidelity with other men as well.

Amber's people say Johnny didn't get medical help for nearly 24 hours, so doctors could not reattach the fingertip ... they used a flap from his hand to sew on a new tip.