The finest opal ever unearthed will be publicly displayed for the first time at the South Australian Museum in Adelaide as part of an exhibition created to celebrate this year's centenary of opal mining in Australia.

Museum director Brian Oldman said the museum had gathered some of the most unique and fascinating opals from around the world to complement its own collection.

The exhibition's centrepiece is the Virgin Rainbow, which is valued at more than $1 million.

"This is an exhibition literally millions of years in the making, because these opals were formed back when dinosaurs walked the earth and central Australia was an inland sea," Mr Oldman said.

"Our north foyer will be transformed to represent this ancient sea and we will also recreate an underground opal mine within the Museum, with dirt brought down from Coober Pedy."

Titled Opals, the exhibition will open on September 25, 2015.