This is the Crown that will be worn for the actual moment of coronation, but why won't he wear it for the entire coronation?

This is the St. Edwards Crown that was created in 1661 for King Charles II. King Charles III will only wear it for the exact moment of coronation because it is extremely heavy. It is solid gold.

He will then don the Imperial State Crown for the rest of the coronation and appear on Buckingham Palace balcony with the Imperial State Crown. It is the more commonly used crown and was made for Queen Victoria in 1838 for her coronation. It contains the fourth largest diamond in the world, the Cullinan II Diamond weighing 317.4 carats.