Your question assumes that a diamond has changed hands for money. Many important diamonds, however, have never been sold, but have been owned based on the stone being a spoil of war or a gift to royalty.
The 'most expensive diamond' may be in regal crown jewels owned by a nation state. The British and Thai crown jewels are examples of magnificent diamonds held this way. Because they have not changed hands for money, there are essentially and otherwise priceless.
There are, however, a few examples of expensive diamonds, some with their record per-carat prices:
yes not only that but the most expensive gem of all at more than $200 million
The Hope is not the most expensive diamond in the world, because its purchase price remains confidential. Other large, rare and famous diamonds have changed hands, but not necessarily for money.
Probably a red diamond, since it is the most rare natural colour.
Red diamonds are the most expensive in terms of money per carat, since red is the rarest colour of natural diamonds.
The Wittelsbach, a 35.56-carat blue diamond mined in India, sold December 2008 at a Christie's auction for $24.3 million.
united kingdom
yes not only that but the most expensive gem of all at more than $200 million
mam diamond
It is the Princies Chrities Diamond.
The Hope is not the most expensive diamond in the world, because its purchase price remains confidential. Other large, rare and famous diamonds have changed hands, but not necessarily for money.
diamond
Hope dimond
Probably not, because it is not a named diamond, so no value for it is available publicly.
The most expensive diamond, by definition, is priceless.
Red diamonds - are the rarest and most expensive.
I think that would be the Hope Diamond. It's not the biggest or most expensive, but over the years it's had a lot of press.