As GLEE star LEA MICHELE emerged in public for the first time since the July 13 death of her boyfriend and co-star Cory Monteith, The ENQUIRER has discovered a stunning secret behind the tragedy – he’d bought her an engagement ring!

A source reveals that days before his fatal overdose, Cory purchased the diamond sparkler with a plan to give it to Lea on her birthday, Aug. 29.

BUT his hopes were dashed by his untimely death.

In addition, Lea had reportedly warned him that, despite her desire to marry him, he had to get a handle on his substance-abuse problems.

“Cory and Lea had been so much in love, and he was looking forward to her birthday because he’d bought her the best present ever,” said an insider. “He told us he was going to propose, but he never got the chance.”

Meanwhile, on July 29, Lea was finally spotted in public, shielded by dark sunglasses and wearing a rhinestone “Cory” necklace. An eyewitness says the 26-year-old actress seemed subdued as she rode in a friend’s car to a house in Bel-Air.

“Lea sat in the passenger seat and looked somber, but she briefly smiled as they talked,” the eyewitness told The ENQUIRER. Other cars arrived at the house at about the same time, and Lea stayed there for about three hours before leaving, according to the source.

That same day, Lea broke her silence about 31-year-old Cory’s shocking passing, posting online an intimate photo of the couple with the caption: “Thank you all for helping me through this time with your enormous love & support. Cory will forever be in my heart.”