Red Diamonds are approximately USD $2-2.5 million/carat. Just like regular diamonds, the price may vary by clarity of the actual stone.
Natural red diamonds are rare enough that your best answer will come from a certified gemologist who can survey the diamond, document it and give you a valuation. You can expect to pay for this certificate. As an alternative, you can commission the manufacture of a man-made red diamond, below.
All else being equal: carat weight, cut, and clarity, a red diamond will always be more expensive than a white diamond.
Jadeite USD $3 + Million/Carat, immediately followed by the red diamond
Red diamonds are rare. The most famous red diamond, the Moussaieff Red Diamond is currently valued at US$7 million; it's a 5.11 carat stone, and the largest red diamond ever rated by the Gemological Institute of America. Read more, below.
Red diamonds are the most expensive in terms of money per carat, since red is the rarest colour of natural diamonds.
Not that much
Depending on the size, cut, clarity, and color of the diamond it can range anywhere from $100 to thousands of dollars, in some cases millions. For the person seeking to buy a 1 carat white diamond, you could spend between US$2,000 to $5,000 or more for the stone.
Colour is only one criteria in the price of a diamond, others being cut, clarity and carat weight. Cut, clarity and carat weight being theoretically equal, a red diamond would probably cost the most, and a black diamond -- Carbonado -- would probably cost least. Since there are dozens of variables in diamond colours, saturations and hues, you can read more about coloured diamonds, below, and pick a group to price and rank.
On a per carat basis, red diamonds are the most valuable of all diamonds, averaging over $1 Million USD per carat. There are only about 8-10 authenticated red diamonds known. If you mean simply what is the most valuable single diamond, then a good guess is the 'Millenium Star', a 203.4 carat perfect white diamond (D color, Flawless). The single most valuable diamond in the world is the 530 carat white diamond, Cullinan I (a.k.a. the Great Star of Africa) in the British Crown Jewels on display in the Tower of London. Although extremely difficult to value - it's not as if such a thing has ever come on the market before - figures most often quoted are in the region of $400 million. The most precious, basis of its importance and value, not based on carat will be The Koh-e-noor, which is now kept in hand by the British Jewels and a part of the British Crown originated from Andhra Pradesh, a state in the Republic of India. Its price cannot be valued and its calculated aproximately as 3.5 times the wealth of the whole world.
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A diamond. That is red.
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