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.

The 44-year-old kept out Burkina Faso's last two spot-kicks to win the shootout for Egypt in Wednesday's semi-final.
"I had a feeling that I was going to save the final two penalties," he said.
"When we went to the shootout, our 2006 title win crossed my mind," he added, recalling his penalty shootout save from Didier Drogba to beat Ivory Coast.
El Hadary is playing at his seventh Nations Cup and has won four titles.
At this tournament he became the oldest player at a major continental championship, and on Wednesday he made his 150th appearance for the Pharaohs.
It is remarkable to think he arrived in Gabon as one of Egypt's back-up goalkeepers to first-choice Ahmed El Shenawy, but injuries meant he was brought in 25 minutes into Egypt's first game.
Since then he has been formidable, conceding only one goal from normal play - in Wednesday's semi-final when Burkina Faso equalised in the second half.
And now a place in Sunday's final in Libreville, where Egypt face either Cameroon or Ghana, offers him a chance to become the first player to win five Nations Cup titles.
His first title came as a reserve in 1988, just two years after his debut, but he was at the peak of his career as Egypt won three back-to-back Nations Cup titles between 2006 and 2010.
"I'm happy to be in contention for all of these records but the most important is for Egypt to win the cup," El Hadary said.
"If we win the title, then the record will pass to me and I will be grateful. But I've never worried about my personal records. The country comes first, then the record book."
It would also be a record-extending eighth title For Egypt, who are already the most successful country in the history of the Nations Cup side - and an incredible return to glory after they failed to qualify for the last three tournaments.
A "substantial reward" has been offered after heirlooms, including a ring given by Charles II to a mistress, were stolen from a stately home.
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Egypt goalkeeper Essam El Hadary foresaw that he would save the penalties that took his side through to the Africa Cup of Nations final.