Million-dollar diamonds up for auction in Geneva

Diamonds might be a girl's best friend, but she'll need deep pockets for this one.

Sotheby's estimates it will sell for 5.6 M to 6.5 million Swiss Francs ($6.1 M - $7.1 M).

Weighing over 100 carats, the Fancy Vivid Yellow Cartier diamond brooch will be auctioned in Geneva on 14 May.

It's known as the Allnatt diamond.

"I'm holding here is the wonderful Allnatt diamond mounted by Cartier in 1952. We call it the Allnatt because the first known owner of the stone was a Major Alfred Ernest Allnatt, a British sportsman, collector of Old Master paintings and philanthropist. He brought this stone to Cartier in 1952, where they put together this beautiful flower-inspired mount," says Catharine Becket, head of sales of Magnificent Jewels at Sotheby's.

The stone is believed to have been mined in South Africa more than 100 years ago.

It is of such importance that it has appeared in international exhibitions, including at the Smithsonian Museum in Washington DC.

This is the first time it's been up for sale in almost 30 years.

"The stone itself weighs 101.29 carats, is fancy vivid yellows, the very top color, and is BS-2 clarity -- so a high clarity. What's particularly notable about this stone is the high crown, as you may see it here, it's an older cutting style," says Becket.

The Allnatt is not the only big money jewel on sale at the Sotheby’s Magnificent Jewels and Noble Jewels Sale.

An Internally Flawless, E colour diamond weighing 37.61 carats is expected to fetch between 1,460,000 CHF – 2,370,000 CHF ($1.6 M - $2.6 M).

In total, there are around 120 lots, many with six or seven figure estimates.

The auction will take place on 14 May at the Mandarin Oriental hotel in Geneva.