Mary-Kate Olsen got a pretty huge present from her BF, Max Snow, for her 21st birthday -- a diamond ring!

But don't get ahead of yourself -- it's a "promise ring," In Touch weekly reports. The bauble is a Snow family heirloom with rose-cut diamonds in platinum. “Mary-Kate and Max are very serious,” a source tells the mag. “They’re practically living together and will probably get engaged before the end of the year.”

Sister Ashley needs to start looking for a man if she wants to have a twin wedding! (JK -- how creepy would that be?!)