[sfg]

Does it have to go to any country? Duleep Singh was the owner, when the British took it. Who is his legal heir?

[mr_toad]

He had eight children (in exile in England), but none of them had heirs.

[xuki]

So we do it British style, go up the family tree and find the nearest cousin or whatever.

[throwawaylinux]

You die without an heir, your property reverts to the state. Looks like the diamond is already with its true owner.

[throwawaylinux]

The British crown is the legal owner now, so bringing the law into it doesn't help you. It's not a legal matter it's a moral one, and OP made a very good point about why that's more difficult than you might think.