Burglars stole three sets of diamond jewellery from one of Europe's largest treasures, the Dresden Green Vault in eastern Germany.
The historic sets each include 37 parts, and there are fears that the thief might try to break them down.
Officials are still trying to determine exactly how much was stolen in the break-in early on Monday.
Saxony's ruler, Augustus the Strong, created the series in 1723 in one of the oldest museums in the world.
"Three out of every ten diamonds have disappeared," said Marion Ackermann, head of the Dresden National Museum.
Police are examining CCTV footage which shows two suspects in the dark. But more people may have been involved in the robbery.