The home of eccentric copper heiress Huguette Clark has already turned out a trove of valuable items – a collection of jewellery including a rare pink 9-carat diamond fetched a total of $21 million at a Manhattan auction last year.

But three years after her death, aged 104, Huguette Clark's sprawling 42-room apartment on Manhattan's Fifth Avenue has just revealed its rarest treasure yet.

A 1731 Stradivarius violin known as "The Kreutzer" after its former owner, French violinist Rodolphe Kreutzer, has been discovered in a closet, and is now expected to auction for between $7.5 million and $10m, the New York Post reported.