The singer recently got engaged to Nickelback frontman Chad Kroeger, who gave her the jewellery piece.
Jeweller Michael O'Connor told US Weekly: "A pear shape diamond set in platinum is a nod to vintage glamour and individuality - perfect for a rocker like Avril.
"The half-moon diamonds are also rare since they would have to be specially cut to fit on either side of the centre diamond. The centre diamond is approximately 10 carats, with an additional four carats of total side diamonds."
Lavigne and Kroeger - who have been dating for around 6 months - celebrated their engagement by partying in Las Vegas.
The marriage will be the second for Lavigne, having wed Sum 41 singer Deryck Whibley in July 2006 but split three years later. It is Kroeger's first.