The Koh-i-noor diamond will not be used during King Charles III's coronation because it is a controversial gem acquired during the age of Empire. It was seized by the East India Company after its victory in the Second Anglo-Sikh War of 1849, and has been part of the Crown Jewels ever since. However, countries including India, Pakistan, Iran and Afghanistan have all claimed ownership of the diamond.
By not using the Koh-i-noor diamond, Buckingham Palace is hoping to avoid any diplomatic row with these countries. The diamond is a sensitive issue for many people in South Asia, who see it as a symbol of British colonialism. Using it at the coronation could be seen as a provocation, and could damage relations between the UK and these countries.
In addition, there is a superstition surrounding the Koh-i-noor diamond. It is said that any woman who wears it will be cursed. This superstition is thought to have originated in India, and it is one of the reasons why the diamond has never been worn by a British queen regnant.
For all of these reasons, it is unlikely that the Koh-i-noor diamond will ever be used at a coronation again