Bonhams will spotlight a rare square-cut, 5.03-carat, fancy-pink, VS1-clarity diamond as well as a tiara owned by the family of Winston Churchill, at the auction house’s fine-jewelry sale in London in late September. “It is extremely unusual to find a pure pink diamond, of even saturation, with no secondary component colors, weighing over 5 carats,” said Emily Barber, Bonhams UK director of jewelry. The auctioneer valued the stone (pictured) at approximately $777,380 to $1 million (GBP 600,000 to GBP 800,000). Also going on sale is a fancy-colored, three-stone diamond ring, featuring an old-brilliant-cut, 0.95-carat, fancy-blue center stone, surrounded by two marquise-cut, 0.51-carat and 0.56-carat, fancy-yellow-orange diamonds. The presale estimate for the piece is about $103,650 to $155,480 (GBP 80,000 to GBP 120,000). Other notable pieces include two diamond tiaras: a geometric, Art Deco tiara/necklace/brooch combination by Hennell, belonging to the Churchill family and esti...