But when news broke of billionaire James Packer's proposal to singer Mariah Carey with a 35-carat, emerald-cut sparkler a couple of weeks later, his supply of the cut between one to two carats was wiped out.

"It came as a shock. I expected to sell about one or two a month, but they're all gone," said Mr Hashem, owner of Diamond Emporium.

"Consumers are definitely influenced by celebrity culture and what celebrities are doing. Without a doubt you do see a spike in inquiries and demand about certain cuts and designs."

Australian jewellers say recent celebrity and royal engagement rings have triggered greater demand for bigger diamonds, coloured dominant stones, and thinner bands.

Mr Hashem, who offers an online customisation service, said the average Australian man splashed between $8000-9000 on an engagement ring, about double the figure 10 years ago.

He said in recent years, actress Jessica Alba's cushion cut diamond, actress Blake Lively's giant, oval-shaped rock, and singer Beyonce's emerald-cut eye-popper had sparked the greatest number of inquiries.

"Larger diamonds (from 1.7 carats) are becoming the preference. Budgets aside, people are comfortable having a size that 10 years ago was considered a bit ostentatious," he said.

Laura Sawade from wedding ring supplier Peter W Beck said according to a Brides To Be survey, the average spend on an engagement ring was $6934 in 2014-15.

She has noticed a strong move towards coloured stones being used as the dominant stone, whether diamond or non-diamond.

"It changed when [Duchess] Kate Middleton got engaged with the sapphire and diamond ring. It made other stones more acceptable," said Ms Sawade, also the vice president of Jewellers Association of Australia.

"Pink and yellow diamonds are very popular. Black diamonds as well. We're seeing a lot of emeralds, rubies and sapphires being used as the dominant stone."

The popularity of the princess cut had slightly waned and the average width of the band had dropped by a millimetre, but she said there would always be a demand for both "incredible classics" and "unique designs".

Georgina Brujic, the newly-installed chief executive of high-end jewellery house Fairfax and Roberts, said she encouraged couples to keep their ring as unique and personal as possible.

"One thing we say to every customers is: 'Find the one that suits'. You'll find your personality in the right ring, which is far more important than what Packer gave Mariah Carey," she said.

"Sometimes you'll find the bigger one isn't the best one because it doesn't suit."

Fairfax and Roberts, recognised as Australia's oldest jewellery house, is home to the "fancy intense" yellow diamond ring, which has a price tag of $412,600.

Ms Brujic revealed its bespoke engagement rings averaged about $40,000-$50,000, though some were sold for $8000.

She agreed that big and colourful stones were back in favour, which to her indicated that people were designing rings that better suited their personality.

"People are also focusing more on quality, rather than the biggest for the best possible price. They're more knowledgeable and asking a lot more questions," she said.

"My personal favourite celebrity ring is Mary-Kate Olsen's one. It's a 1950s Cartier and it's the most gorgeous ring I've ever seen in my life. It's stunning, just stunning."