Newt's Emerald

On her eighteenth birthday, Lady Truthful, nicknamed " Newt, " will inherit her family' s treasure: the Newington Emerald. A dazzling heart-shaped gem, the Emerald also bestows its wearer with magical powers.
When the Emerald disappears one stormy night, Newt sets off to recover it.   Her plan entails dressing up as a man, mustache included, as no well-bred young lady should be seen out and about on her own. While in disguise, Newt encounters the handsome but shrewd Major Harnett, who volunteers to help find the missing Emerald under the assumption that she is a man. Once she and her unsuspecting ally are caught up in a dangerous adventure that includes an evil sorceress, Newt realizes that something else is afoot: the beating of her heart.
In Newt' s Emerald, the bestselling author of Sabriel, Garth Nix, takes a waggish approach to the forever popular Regency romance and presents a charmed world where everyone has something to hide.