Credit: Jeffrey Mayer/; Steve Granitz/

Fact of life: Brad Pitt and Angelina Jolie really value their pal, 80s star Mindy Cohn.

Last week, Us Weekly broke the news of their secret friendship, reporting that the Facts of Life alum, 44, is godmother to two of the couple's six kids and that the power couple even gave her a "huge diamond ring to thank her."

Now, an insider tells the ring is utterly "unbelievable," describing it as "four or five carats."

Says the source, "She wears it on her right finger."

When the awe-struck source asked Cohn about the rock -- solitaire with diamonds all around in a white gold setting -- Cohn replied: "'Brad and Angelina gave it to me.'"

Brangelina's generosity doesn't end there.

"She said she visits [the Jolie-Pitts] wherever they go," the source tells Us. "They pay for her airline ticket."

So, how did they become BFFs anyway?

Cohn (who is pals with Jolie's brother James Haven) has "known Brad for years for years," the source tells Us. "They are very close."