Emerging from a cloak of secrecy and legal fog, a trove of storied imperial gemstones is about to go on display in Canada, the improbable country where they lay hidden for more than 80 years. The Habsburg family jewels, including the 137-carat Florentine Diamond, were smuggled out of Europe at the outset of the Second World War by Zita of Bourbon-Parma, the last Empress of Austria and Queen of Hungary, and were presumed lost for decades. Only last year did her grandchildren reveal that, all along, they had been stashed in a vault in Quebec – the same province where...