A Florida mom is accused of killing her 3-month-old son ... and cops say one of the biggest clues may have been sitting right on her finger.

Jessica Vlasac was arrested Thursday after investigators say her baby suffered multiple blows to the head -- including injuries consistent with being struck by the large, square-shaped ring she was wearing.

Cops say Vlasac initially claimed she'd accidentally fallen asleep with the infant and woke up to find him face down and unresponsive.

She allegedly told the father of the child, "I think I killed our son" ... but authorities say the autopsy told a much darker story.

The medical examiner found bleeding around the skull and three separate impact injuries to the baby's head ... including an L-shaped wound consistent with a square-like object.

Enter Vlasac's ring -- detectives noticed she was wearing a large, diamond-like square ring and later seized it.

Cops say Vlasac asked them whether the ring killed the baby ... and the medical examiner later said the injuries were consistent with being struck by a ring.

Investigators say the baby's father also showed them messages indicating she bought alcoholic shots earlier that day ... he told cops she gets violent when she drinks and that she asked him not to show detectives the messages.

Authorities say co-sleeping did not cause the death ... Vlasac was booked on aggravated child abuse resulting in death.