A rare rough ruby dubbed one of the world's biggest has gone on display for the first time ahead of an auction in the Gulf emirate of Dubai. 

The 8,400-carat stone -- nicknamed Burj Alhamal and weighing 2.8 kilogrammes (over six pounds) -- was mined in Tanzania and made its debut at a Dubai hotel on Friday as part of SJ Gold and Diamond's Callisto collection.

The company says it is "one of the biggest" rough rubies in the world, with managing director Patrick Pilati calling it among the "rarest rubies ever found". 

He said the Tanzanian stone was "not heated", meaning "it has not been treated, so it's natural and that's why it's precious."

Local reports have suggested the rough ruby with greenish and dark-purple hues could fetch up to $120 million. 

The stone will be on display at different locations in Dubai for the next 30 days before going to auction, Pilati told AFP.

Late last month, a giant diamond nicknamed "The Rock" went on display for the first time in Dubai.

The 228.31-carat pear-shaped gem, which was mined and polished in South Africa more than 20 years ago, is the largest white diamond ever to come to auction, according to Christie's.

The diamond is expected to fetch over $30 million.