Casey Johnson has some explaining to do: The heiress to the Johnson & Johnson Band-Aid fortune is accused of using a faux diamond she once presented to an ex-girlfriend to propose to the troubled reality TV star Tila Tequila.

The former MySpace pin-up stunned fans by announcing her engagement to the Johnson & Johnson heiress via video posting website Ustream.com last Wednesday morning, but Johnson’s ex lover, socialite Courtenay Semel, claims the “17-carat diamond ring” is a hand-me-down – insisting Johnson gave her the same ring while they were dating earlier this year.

“I can confirm that the engagement ring given to Tequila by Johnson was not purchased for her. It has previously been worn by me, a statement I can back up with photographic evidence,” Semel tells RadarOnline.com. She adds, “It is absolutely not real and Casey is well aware. I would not expect Tila to be able to differentiate a real diamond from a fake one. Her ring is as real as her engagement!”