Christie's will offer a 24-carat pink diamond set in a Bulgari ring at its Magnificent Jewels sale in Geneva on Tuesday, Nov. 10, with an estimate of CHF 10 million to CHF 15 million (about US$12 million to US$18 million), the top lot of the city's autumn jewelry auctions.

The sales are the last big price check of the year for colored diamonds and royal-provenance gems and the prices achieved will serve as reference points for collectors and dealers in Hong Kong, Singapore and elsewhere in Asia.

Christie's bets on a never-auctioned Bulgari Pink

The stone is an emerald-cut, fancy intense pink diamond classified as Type IIa, the chemically purest category of natural diamond, which accounts for less than 2% of diamonds found, according to National Jeweler. It has been held privately for more than 70 years and has never been offered at auction.

It comes from the collection of a European noble family. Family accounts say the current owner's great-grandfather bought the rough stone in India in the 1950s and took it to Bulgari, then a favorite of film stars working in Rome, to be set as a ring, National Jeweler reported.

Christie's points to the color, the rectangular step cut and the size as the source of its rarity. The auction house also leans on the ownership record, describing an "unbroken history of ownership." The sale takes place at the Four Seasons Hotel des Bergues, with viewing from Nov. 5 to Nov. 10.

A pink diamond of this size and saturation rarely appears in public. That makes the low estimate the figure to watch: a sale near CHF 10 million would suggest buyers are still cautious on large colored stones, while bidding past the high end would point to deep demand at the top of the market.

Christie's is also offering a Van Cleef & Arpels Oiseau Mystérieux ruby and diamond brooch in its November sales, according to Galerie magazine.

Sotheby's leans on Romanov and Cartier history

Sotheby's follows on Wednesday, Nov. 11, with its Royal & Noble Jewels sale of 78 lots, according to the auction house's website. Previews run at the Mandarin Oriental in Geneva from Nov. 6 to Nov. 10.

The lead lot is the Romanov sapphire, an unheated Ceylon stone of 197.80 carats estimated at CHF 850,000 to CHF 1.7 million, National Jeweler reported. Fabergé mounted it, and Dowager Empress Maria Feodorovna wore it at the Winter Palace ball of 1903. After the 1917 revolution it was listed in the Fersman inventory of imperial treasures and later sold off by the Soviet government.

The stone then passed to opera singer Ganna Walska before Cartier acquired it. In 2015, Cartier made the transformable bracelet that holds it today, with an engraved rock crystal set with diamonds that can be swapped in for the sapphire.

Cartier also figures in the second big lot. A transformable black opal and diamond tiara, made in London in the late 1930s for Mary Cavendish, Duchess of Devonshire, carries an estimate of CHF 780,000 to CHF 1.1 million. The duchess wore it at Queen Elizabeth II's coronation in 1953, Sotheby's said.

Other lots include a Mellerio diamond tiara shown at the 1862 International Exhibition, estimated at CHF 300,000 to CHF 500,000, and a ring pairing a 7.83-carat Kashmir sapphire with a 6.77-carat Colombian emerald, estimated at CHF 350,000 to CHF 500,000. A bangle centered on a 27.38-carat Colombian cabochon emerald, formerly owned by Queen Amélie of Portugal, is estimated at CHF 70,000 to CHF 100,000.

Andres White Correal, Sotheby's chairman of jewelry for Europe and the Middle East, tied the series' results to collectors' "enduring fascination" with royal pieces, National Jeweler reported.

What the results will signal for Asian collectors

Provenance carries much of the weight in both sales. A stone tied to an imperial court or a famous jeweler's archive usually sells for more than an unrecorded gem of similar size and quality. The Romanov sapphire's estimate per carat is modest next to top colored diamonds, which leaves room for a large gap between estimate and hammer if several bidders compete.

Phillips is also selling jewels in Geneva this month, including a pearl and diamond necklace linked to Queen Mary and an emerald and diamond bracelet that belonged to Empress Eugénie, Galerie reported.

For Asian buyers, Geneva is one stop in a cycle that also runs through Hong Kong, where Christie's has offered jadeite and snuff bottles from the Ai Lian Tang collection. Retail demand for new jewelry is a separate gauge, tracked in LVMH's third-quarter watches and jewelry sales.

The Bulgari Pink is offered on Tuesday. Its hammer price, against the CHF 10 million low estimate, is the first number to read when the week's totals come in.