Diamond Necklace remained unbeaten after winning her second classic of the season in the Prix de Diane (G1) June 14.
Trained by Aidan O'Brien, Diamond Necklace was made to work for the narrowest success of her five-race career, getting the better of 34-1 chance Pink Panthera, a contrast to her 3-length win at Longchamp in the Poule d'Essai des Pouliches (G1) last month.
Ridden in midfield by Ryan Moore, the runners were led by her stablemate Moments of Joy before Diamond Necklace found headway in the final 2 furlongs and battled gamely to land another valuable prize.
Moore said: "I was always happy with her; she did everything beautifully. She was great going down; she began well, and she was always where I needed her to be. There was a slight issue when Esna started coming back on me coming round the bend, and I had to start moving a little bit sooner than I had planned.
"The second horse might have headed her at one point, but when I called on her a second time she stuck her head down and straight away took it back again. All the way through the race she never felt like she was going to get beaten.
"I'm still learning about her; it's only the second time I've ridden her. But she's such an uncomplicated filly, and she only does what she needs to do so I'd like to think there'd be more to come when needed.
"She's very straightforward, but she just feels like class the whole way. Her professionalism, the way she goes through the race, her movement and her balance, she's just a classy filly who travels well and has loads of pace when you ask her so I couldn't be happier with her."
David Menuisier picked up a valuable third place through Inis Mor, who ran a big race at odds of 22-1 under Oisin Murphy.
Following her triumph, Diamond Necklace was shortened for multiple ante-post targets. Paddy Power cut her to 4-1 (from 6) for the Irish Oaks (G1), while Betway reduced her to 10-1 (from 18) for the Eclipse (G1) and to 10-1 (from 14) for the Arc (G1).
Stepping beyond a mile for the first time in the 1 mile 2 1/2 furlong contest, Diamond Necklace saw out the trip well, as her pedigree suggested she would, being by French Derby (G1) winner St Mark's Basilica.
The success gave O'Brien a seventh major European classic of the season, further underlining Ballydoyle's domination.
