Mila Kunis stops to buy some flowers on September 4, 2013 in Studio City, California. Credit: Premiere/Rocstar/FAMEFLYNET PICTURES

Could a '70s-themed wedding be on the way? Mila Kunis immediately sparked rumors about a possible engagement to beau Ashton Kutcher when she stepped out in L.A. on Wednesday, Sept. 4, wearing a simple gold band -- on that finger.

Kunis, 30, and Kutcher, 35, first started dating in April 2012, and have been tight-lipped on their relationship ever since. Most recently, the pair spent some time with Kutcher's parents in London in May. The next month, they were caught kissing during a vacation with Princess Beatrice and her boyfriend Dave Clark in Saint Tropez, France.

An engagement ring would be a very special birthday present for Kunis -- the Oz the Great and Powerful actress just turned 30 on Aug. 14.

There's one detail that Kutcher needs to work out if he and Kunis ever walk down the aisle: finalizing his divorce from Demi Moore. The Jobs actor filed for divorce from Moore, 50, in December 2012 after six years of marriage. They previously announced their split a month earlier.