The Diamond of Drury Lane by Julia Golding

Catherine (Cat) Royal was left as a baby on the steps of the Royal Theatre in London and that is how she came to be raised in the theatre. In the winter of 1790, Cat becomes embroiled in a mystery; there is a diamond hidden in the theatre. Lots of rapscallion adventures in this book; I would suggest it to readers who enjoyed Pullman's The Ruby in the Smoke.