Mary-Kate Olsen's engagement ring reportedly cost $81,250.

The actress-turned-designer is set to marry her partner of two years, Olivier Sarkozy - who is the half-brother of former French President Nicolas Sarkozy - after she was spotted flaunting a massive sparkler last week, and it has now emerged the vintage Cartier ring was bought at a Sotheby's auction on February 6.

According StyleCaster, the band boasts a four-carat European-cut diamond surrounded by 16 calibré-cut sapphires and 1.5 carats of diamonds.

It dates back to 1953 and was apparently previously owned by ''a Tennessee collector''.

Mary-Kate flaunted her new accessory for the second time at the Louis Vuitton show at Paris Fashion Week on Tuesday (04.03.14).

The marriage will be Mary-Kate's first and Oliver's second, after his union to Charlotte Bernard, with whom he has two teenage children, Julien and Margot.

The fashionista and the banker live in New York where they have a $6.25 million townhouse, but their union has caused some controversy due to their 17-year age gap.

Mary-Kate previously commented: ''Everyone has an opinion. I find it's better to focus on what's in front of you and to keep putting one foot in front of the other.''