A tiara and more than 40 diamond, sapphire and emerald items were stolen during a break-in at Goodwood House, West Sussex, on 13 January.
The jewellery belonging to Lord and Lady March is said to be irreplaceable.
Police have not given a figure for the reward being offered in return for information about the missing items.
1820 diamond tiara worth £400,000
Det Insp Till Sanderson, of Sussex Police, said his team had been working closely with Lord and Lady March and the estate to trace and identify the people responsible for the theft of this "treasured property".
"I hope the offer of a substantial reward by the insurers, for information leading to the recovery of important items of jewellery and personal effects of historical significance, will encourage anyone who knows anything to come forward," he added.
A 26-year-old man from Hampshire who was arrested in connection with the raid has been bailed until February.
A "substantial reward" has been offered after heirlooms, including a ring given by Charles II to a mistress, were stolen from a stately home.