Newly engaged singer Mariah Carey is bearing one of the biggest and rarest diamonds on her finger, according to jewellery designer Ben Bunda.

The founder and Creative Director of Bunda Fine Jewels says although the value of the 35-carat diamond ring Australian billionaire James Packer gave to Carey is unknown, it is estimated to be around the $5 million mark.

"This is a very special ring, as a 35-carat diamond is incredibly rare," Bunda told AAP.

"In New York in December 2015 there was a sale of a 38.27ct Square Emerald Cut diamond ring for the value of $4,058,000 USD ($6,268,536 NZD).


"There is a very small production of significant size rough diamonds mined globally each the rarity of such a large stone adds to value."

The expert added, Packer's ring choice wasn't only about the size, but also the ring's clarity.

"Very large diamonds like this often have very high clarity."

"So it's highly likely that James Packer has bought Mariah Carey a ring not only of exceptional size, but also what looks to be exceptional clarity, cut and colour."

Packer popped the question last week but there is no word yet on a date for the wedding. If the ring is anything to go by then there is no doubt the celebration with be extravagant.