Tennis world number one Aryna Sabalenka has announced her engagement to partner Georgios Franguli, but it’s her massive diamond ring that has drawn almost as much attention as the happy news.

Sabalenka, who has been dating Franguli for almost two years, shared photos of the couple celebrating their engagement online this week.

The tennis champion showed off her striking 12-carat diamond ring, custom-designed over several months by US jeweller Isabela Grutman and Franguli.

According to diamond expert Nick Ireland, the ring could be worth about $US1 million (around $1.4 million), with similar stones potentially fetching even more in Australia.

Ireland told Nine the centre diamond alone is substantial.

“The main diamond is 12-carats, then you have possibly up to a carat of diamonds on the band itself,” Ireland said.

“Obviously, if it’s a natural diamond of high colour and high clarity, it could easily be more than a $US1 million… reaching up to $US1.5 million.”

The design also includes a small but meaningful detail, a halo set with emeralds – Sabalenka birthstone – hidden beneath the centre stone.

Ireland said the feature is unusual and adds a personal element.

“Having emeralds in a little hidden halo underneath … you wouldn’t just see that sort of combination,” Ireland said.

“It certainly wouldn’t just be sitting there on the shelf ready to go.

“The emeralds are only about 1.5mm in size maximum, they’re not [worth] a huge amount, but they appear to be very high quality, very high colour.

“It’s a nice personal touch.”

Grutman confirmed the emerald detail was Franguli’s idea, telling People it helped make the piece distinctive.

“It’s a personal touch that makes the ring uniquely hers,” she told the outlet.

In an Instagram video, the jeweller also shared a glimpse of the design process.

”I’m creating a very special ring for a very special person, and I’m so excited about it,” she said.

“I’m analysing this 12-carat oval diamond … When I have the future husband pick the perfect ring and stones, I analyse what she likes.

“Is she more modern? Is she more classic? What is her personality?”

“This stone is perfect for this woman I’m designing for.”

The video also shows the ring up close, including the hidden emerald halo and diamond-set claws holding the large centre stone in place.

While a natural diamond of that size places the ring firmly in the luxury category, Ireland said there are more affordable alternatives for couples inspired by the look.

A 12-carat lab-grown diamond, he said, could cost between $15,000 and $20,000, a fraction of the price of a natural stone.

Images: Instagram