All else being equal: carat weight, cut, and clarity, a blue diamond will always be more expensive than a white diamond.
Stone for stone, a diamond is more expensive than an emerald.
Platinum is not more expensive than a beautiful, natural diamond.
Mined or natural diamonds are more expensive than man-made diamonds. If monetary value is an advantage, then a mined diamond has the advantage of value.
Gram for gram, a gem-quality diamond is always more expensive than platinum. However, gram for gram, platinum is always more expensive that industrial diamonds. It is important to keep in mind that diamond prices are kept artificially high because DeBeers hides away much of the diamonds they mine to exaggerate scarcity i.e. diamonds are a manipulated market (vs. a free market).
At a first glance, this seems really weird since:oxygen is essential for human survivaldiamond is not essential for human survival...and therefore, oxygen should be expensive whereas diamond not.However, looking at the scarcity we realize that there are a lot more oxygen than there are diamonds.Therefore, since oxygen is abundant (while not infinite), it is cheap (unit price). and since diamond is extremely scarce, it is expensive.
All else being equal: carat weight, cut, and clarity, a red diamond will always be more expensive than a white diamond.
blue
A natural blue diamond will always be worth more than a white or colourless diamond, given equal carat weight, clarity, and excellence of the cut.
Stone for stone, a diamond is more expensive than an emerald.
it will glow white, or more than likely, blue
Every diamond is valued by its clarity, colour, carat weight and cut. Compared on an equal basis, you will pay less for a black diamond.
Platinum is not more expensive than a beautiful, natural diamond.
Given the same carat weight, clarity and cut, you will pay more for a pink diamond than for a 'white' or colourless diamond.
By weight, when comparing equal amounts of platinum and diamond, gem-quality diamond will always be more expensive.
Carat for carat, no gem is more expensive than a diamond, given a superior cut, clarity and colour classification. Other gems can be more expensive than diamonds when the diamonds lack top qualities.
Any diamond is valued by its cut, colour, clarity and carat weight. Blue diamonds are the most rare of the naturally coloured stones. It is possible to treat a diamond and turn its colour to blue, but this fact must accompany the sale of any such stone. A natural blue diamond of just over seven carats in weight sold at auction for more than US eight million dollars, setting a record. (Link, below.) Smaller and especially irradiated or treated diamonds will sell for much less. A local gemologist can evaluate your diamond and document its value.
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:The world's most expensive diamond -- 35.56 carat Whittlesbach blue -- sold for $24.3 million, beating a previous record of $16.5 million for a 100-carat diamond in 1995.The DeBeers Millennium Star DiamondThe Taylor-Burton DiamondThe CentenaryCullinan (never sold, gifted to the British Crown)Hope DiamondA 7.03 blue diamond sold for £6m ($9.5 million) in March 2009 in Geneva.Chopard blue diamond ring for 16,260,000 dollars. It has a 9-carat blue diamond, more white diamonds, and a 18k white gold setting. (In general, blue diamonds are the among rarest and most expensive diamonds in the world.)