Stewart performed for the couple at the VIP bash in Philadelphia - before being introduced backstage.
And he admits to being blown away by the 59-year-old's looks.
The 62-year-old singer says: "She always looks glamorous to me. I can see what he sees in her, quite honestly, between you, me and the lamppost, reports Irish Examiner.
According to Times Online, consisting of 36 rubies interlaced with diamonds, the necklace is estimated to be worth several million pounds, but royal aides declined to say who the donor was. The piece has not been seen before; the Duchess generally favours more understated jewellery.
“When people give presents it’s on the understanding that it’s private. It’s the first time she’s worn it. She very much likes it,” the Duchess’s spokesman said. The royal couple have been on only a handful of official overseas tours together since marrying in 2005; they have visited Egypt, Saudi Arabia, India, Pakistan and the US .
A Clarence House spokesman said the duchess was particularly fond of the piece. He said: "She was given it in on an official tour. We can't confirm who by."
Camilla joined Charles at the Academy of Music in Philadelphia for its 150th anniversary where they watched guest star Stewart. The rocker said of the necklace after meeting the royals backstage: "It was gorgeous. She always looks glamorous to me. I can see what he sees in her quite honestly, between you, me and the lamp post."
Earlier in the day, Camilla had worn a distinctive heart-shaped brooch made of diamonds and two sapphire droplets.
It used to belong to the Prince's beloved grandmother, the Queen Mother, informs Manchester Evening News.