Mary-Kate Olsen has been shopping for an engagement ring.

The 27-year-old fashion designer and her 44-year-old beau Olivier Sarkozy - whom she has been dating for over a year - have been spotted checking out prices on marital jewellery at the Neil Lane boutique in Los Angeles on several occasions in last month, which has sparked speculation they are about to get engaged.

A source told the New York Post newspaper: ''Mary-Kate has been seen several times in the last three weeks checking out huge engagement rings at the Neil Lane store.

''She really likes cushion-cut diamonds, and all of the rings she has looked at are over five carats. She has narrowed it down, but still hasn't made a final decision.''

The blonde beauty initially had her eyes on a platinum-setting ring, but had no concerns when it came to money as it had a price tag of over $100,000 attached to it.

Another source said: ''I don't think price was an issue.''

Mary-Kate previously dated artist Nate Lowman for over two years before splitting in 2010, while Olivier romanced actress Stella Schnabel for a year before their break-up last March.

Olivier was previously married to wife Charlotte and they have two children together.

Charlotte has since called the coupling ''grotesque'' and ''not right'' due to their age differences.