It was a very Happy 22nd birthday indeed for Lindsay Lohan, who celebrated the milestone with a new $22,000 Cartier diamond ring from lady love Samantha Ronson

“Lindsay is thrilled and she and Sam are very content,” one tabloid insider spills. “It’s not an engagement ring – simply a token of her commitment.”

The spy adds, “Sam and Lindsay are inseparable. Sam has been a really good influence on Lindsay. There is no one else special in Lindsay’s life apart from Sam….It’s really sweet that Lindsay has found someone she can trust and rely on.”