One can also add with the Koh-I-Noor that in 2016 the solicitor general of India Ranjit Kumar before the Supreme Court of India stated that the diamond “was given voluntarily by Ranjit Singh to the British as compensation for help in the Sikh Wars. The Koh-i-Noor is not a stolen object”. While the court eventually only ruled on its jurisdiction the government has made its position clear.

The Pakistan claim has only been lightly pursued with a 1976 letter to the PM and response highlighting the treaty of Lahore. The aphganistan claim is difficult as it was likely originally from the area of aphganistan but the wars over the centuries probably remove the true claim and the fact that the taliban are the claimants who are currently asking it is not being given any true high level thought I should think.

Ultimately as you said it probably is the only gem with a genuine reason for the reputation but also the claims against it are slowly diminishing as the country’s or at least their governments take the position that is was gifted or not worth pursuing in any real terms as in the end the diamond while significant is not the most important cultural artefact that should be returned especially considering how it was one of the ‘less murderous’ acquisition with a treaty actually being signed.