Kim Kardashian reportedly picked out her engagement ring herself.

The reality TV star's rapper beau Kanye West popped the question to her on Monday (21.10.13) - the same day as her 33rd birthday - but it appears the brunette beauty knew the proposal was coming as she selected the lavish 15-carat diamond ring.

A source told RadarOnline.com: ''Kim had picked out her own Lorraine Schwartz engagement ring.

''[She] told Kanye to get the ring from Lorraine Schwartz because she loves the jeweller's diamonds. She knew the proposal was happening, she just didn't know when it would happen.''

The 'Keeping Up with the Kardashians' star's engagement ring could be worth an eye-popping $7 to 8 million because the diamond has been flawlessly cut, according to expert jeweller Adelaide Polk-Bauman.

The diamond specialist told the New York Daily News newspaper: ''This is exceptionally rare. Very few places in the world have diamonds such as that.''

In comparison, Kim's previous engagement ring - gifted to her by ex-husband Kris Humphries in 2011 - is supposedly worth $2 million, despite boasting a 16.2 carat diamond.

A source added: ''Before Kim gave the ultimate seal of approval on the sparkler, Kanye looked at over 20 diamonds. The diamond has no flaws and has more sparkle than Kim's previous ring.''

Kanye proposed to Kim - the mother of his four-month-old daughter North - by hiring out the AT&T Park baseball stadium in San Francisco.

The 'Black Skinhead' rapper invited Kim's friends and family to celebrate the occasion, which included a fireworks display.