A RARE yellow diamond discovered in South Africa last year is expected to fetch up to R100-million when it goes on auction next month.

Called the Sun-Drop, the 110.03-carat gem has wowed not only those lucky enough to catch a glimpse of it, but even experts tasked with grading it.

John King, the chief quality officer of the Gemological Institute of America (GIA), said from California this week that it was "one of the most beautiful and rarest I've ever seen".

King has been in the diamond grading industry for more than 30 years and, despite the GIA analysing and grading over two million diamonds a year, described it as a highlight of his career.

He said the Sun-Drop was something he would "definitely remember" after overseeing the process of putting it through rigorous tests.

"The rarity of a naturally occurring diamond like the Sun-Drop is just astronomical ... it has been graded Fancy Vivid Yellow, the highest colour grading for a yellow diamond," said King.

"It's really rare, really unusual at that size, that depth of colour. You don't find those often."

King said whoever bought the diamond could consider themselves privileged.

The Sun-Drop, the world's largest pear-shaped Fancy Vivid Yellow, will be auctioned by Sotheby's in Geneva in November. Spokesman Marie-Beatrice Morin said the diamond was cut and polished by Cora International in New York, one of the world's leading manufacturers and diamond dealers.

Morin said the Sun-Drop "comes to the market with an estimate of between $11-million and $15-million".

The diamond was taken on a three-city tour earlier this year, including being put on display at London's Natural History Museum.