The Star of Kazan -- Eva Ibbotson

An abandoned baby taken in by a cook, a housekeeper and a trio of professors, an inherited trunk full of beautiful old clothes and paste jewels (including a copy of the famous and fabulous Star of Kazan), a run-down estate, a gypsy boy, a secret garden, the Lippizaner stallions and of course, a horrible neighbor:

She was wearing a pleated tartan skirt and a tartan tam-'o-shanter, although the Eggharts were not known for their Scottish blood. Her small dark eyes were set very deep in her pale cheeks, and she had a particular way of walking, with her behind thrust out, as though she wanted people to know that her backside, like her front, was important.

Classic Ibbotson (minus the fantasy). I loved it, even the horses.