Prince Rupert Loewenstein, the Bavarian aristocrat and banker who managed the Rolling Stones' finances over the nearly 40-year span ending in 2007, has passed away at the age of 80.

While they were one of the biggest bands on the planet, the Stones' cash flow was in disarray when Loewenstein took over -- a result of their ugly split with Allen Klein. Loewenstein openly admitted that he'd never heard of the group before being personally introduced to them, yet he did brilliant work on the Rolling Stones' behalf -- turning them from shaggy young rock stars into the incredibly wealthy stewards of a mind-blowingly lucrative brand.

"He is a great financial mind for the market. He plays that like I play guitar," Keith Richards told Fortune in 2002. "He does things like a little oil well. And currency -- you know, Swiss francs in the morning, switch to marks in the afternoon, move to the yen, and by the end of the day, how many dollars? That's his financial genius, his wisdom. Little pieces of paper. As long as there's a smile on Rupert's face, I'm cool."

After giving up his position with the Rolling Stones and retiring in 2007, Loewenstein penned a memoir about his time with the band, titled 'A Prince Among Stones: That Business with The Rolling Stones and Other Adventures.' Frontman Mick Jagger was none too pleased with the book, grumbling that "I don’t think your ex-bank manager should be discussing your financial dealings and personal information in public. ... It just goes to show that well-brought-up people don’t always display good manners." Still, we're sure he got over it the next time he looked at his bank balances; according to the NME, Loewenstein's advice helped Jagger alone amass a fortune worth an estimated $337 million.

Music Week reports that Loewenstein, whose full name was Prince Rupert Ludwig Ferdinand zu Loewenstein-Wertheim-Freudenberg, died on May 20, passing "peacefully ... after a courageous battle against a long illness."