A Fancy Intense Blue diamond, a Fancy Gray-Blue diamond and a Kashmir sapphire ring will lead Bonhams London Fine Jewellery sale this September.

The sale of 188 lots includes a selection of white and coloured diamonds, sapphires from Kashmir, Burma and Sri Lanka, signed jewellery from Cartier and Van Cleef & Arpels, and the largest private collection of jewellery ever to come to auction by Anglo-Italian designer Andrew Grima.

Headlining the sale is A Fancy Intense Blue diamond estimated at £1,200,000-1,500,000. The pear- shape diamond, weighing 4.03 carats, has been in a UK private collection for over 30 years.

According to the Gemmological Institute of America, only one in 10,000 diamonds has a fancy colour, which makes such stones rare and highly sought after. Often referred as "miracles of nature", coloured diamonds have, on average, increased in value by 100 per cent over the past decade with blue stones showing the strongest growth over the past 12 months (+5.5%), according to the Knight Frank Luxury Investment Index*. (*Data to end of Q1 2017 supplied by the Fancy Colour Research Foundation.)

Jean Ghika, Head of Jewellery for Bonhams UK & Europe, said: "We are delighted to bring this exceptional stone to the market. Blue diamonds are extremely rare and highly coveted. We are looking forward to showcasing this stone, along with other selected lots in London, New York, Hong Kong and Geneva, where we anticipate considerable interest."

Staying with coloured diamonds, the second headline lot is A Fancy Coloured Diamond brooch. This features a central square step-cut Fancy Gray-Blue diamond, weighing 4.83 carats, with a Fancy Dark Brown-Greenish Yellow step-cut diamond to either side, weighing 2.51 carats and 2.38 carats respectively, suspending a pear-shape Fancy Pink Diamond drop below, weighing 1.90 carats. The brooch dates from the mid-20th century and has a pre-sale estimate of £650,000-850,000.

The third notable lot in the sale is A Kashmir sapphire single-stone ring, weighing 9.61 carats. The octagonal step-cut sapphire has no indications of heating and is estimated at £500,000–700,000.

Jean Ghika added: "Kashmir sapphires are amongst the most highly prized gems for serious collectors given the mines were only open for a period of approximately 30 years from the late 1880s onwards. This stone has the classic attributes associated with sapphires from the Kashmir region, a rich and intense blue colour coupled with a velvety appearance."

Bonhams will also be auctioning a number of signed pieces of jewellery by Cartier as part of its sale. These include A pair of Art Deco diamond clip brooches by Cartier, circa 1935, estimated at £40,000-60,000.

Adding a further touch of colour to the sale is An emerald and diamond bracelet by Cartier. The bracelet, which dates from the 1960s, features a series of alternating step and octagonal-cut emeralds, accented to either side by brilliant-cut diamonds and has a pre-sale estimate
£50,000-70,000.

For buyers who are seeking white diamonds, the sale will also feature a charming example of an old-cut diamond. The old marquise-cut diamond single-stone ring, weighing 4.61 carats, is D colour, VVS2 clarity, and potentially flawless. It is accompanied by a letter from GIA stating that the diamond is Type IIb. It has a pre-sale estimate of £80,000-100,000.

The sale will also include the largest private collection of jewels by Andrew Grima ever to appear at auction. Grima's groundbreaking designs, which drew inspiration from art, sculpture and the natural world, injected desperately needed originality and glamour into what had become a stagnant jewellery scene in post-war Britain. He created bold and unusual jewels whose value lay in their aesthetic composition rather than just in the carat weight of expensive diamonds and precious gems. He is recognised as one of the great modernist jewellery designers of the 20th century and his pieces are highly sought after worldwide.

The collection features 55 pieces of jewellery designed by Grima. Highlights include pieces from his admired "About Time" watch collection for Omega, one of the most innovative collections of watches ever made, as well as works from the "Rock Revival" collection in that incorporated large, uncut and unusual gemstone crystals.

Full details of the lots to be auctioned at Bonhams London September Fine Jewellery sale will go online at the end of August and can be found here: https://www.bonhams.com/auctions/24173.

The sale commences at 1pm GMT and will take place at Bonhams New Bond Street salesroom in London