The 27-year-old is said to have first visited the Neil Lane jewellery shop – whose creations are a favourite with Hollywood's biggest stars – back in mid-November and has returned several times since.
So it seems the former child star may be coming closer to choosing her dazzling diamond to mark an engagement to boyfriend Olivier Sarkozy, the brother of former French President Nicolas Sarkozy.
According to the New York Post's Page Six, she has her heart set on a rock "over five carats".
A source told the newspaper, "...she really likes cushion-cut diamonds, and all of the rings she has looked at are over 5 carats."
"I don't think price was an issue," they added, noting that Mary-Kate had set aside several rings with price tags of $100,000 or more.
The pint-sized blonde has been dating Olivier, who is 17 years her senior, for more than a year. He has not accompanied her on engagement ring shopping, it has been reported.
The pair have received plenty of media attention since going public with their relationship. It was thought the loved-up couple were engaged earlier this year in January when Mary-Kate was photographed with a ring on her left hand during a trip to Paris.
Shortly after, a source told Us Weekly that the Olsen twin wanted to take her time with the relationship and plans to marry.
Olivier is divorced and has two children with his ex-wife Charlotte Bernard, from whom he split in 2010 after 13 years of marriage.