There isn't any information on the smallest carat diamond ever sold. However, I did find the world's smallest diamond. It weighs in at 0.0003. It is being sent into the Guiness Book of World Records.
The Wittelsbach, a 35.56-carat blue diamond mined in India, sold December 2008 at a Christie's auction for $24.3 million.
First, you'd have to find such a diamond. This size and colour are uncommon, blue being one of the rarest diamond colours. Recently (21Oct2010), the Bulgari Blue Diamond, "...a triangular 10.95 carat Fancy Vivid blue diamond, (the largest such blue diamond of this cut ever offered at auction)," sold for $15.7 million or $1.4 million per carat. (From The Telegraph, London.)
A 507 carat diamond.
The 'costliest diamond of the world' is measured by the price paid for a diamond. Depending on your particular criteria, there are several examples.When money changes hands to represent the costliest diamond...The JCK Marketplace Web site reports that an 84-carat [faceted] diamond sold for US $16.2 million, at "... $191,980 per carat, the highest price per carat ever paid for a white diamond at auction." This auction took place in 2007.At the same auction..."An extremely rare fancy vivid blue pear-shaped diamond, weighing 4.16 cts. and mounted as a ring, ...sold for $4.7 million. The per-carat price was more than $1.1 million, making it the second highest price per carat paid for a colored diamond at auction."In 2010, Reuters reported that "Petra sets record, sells diamond for $35 million" in its headline for the sale of a 507 carat raw diamond.
A one carat diamond ring that costs $4,125 in 2013, sold for approximately $400 in the 1950s.
A one-one-hundredth of a carat diamond is referred to as a chip, usually, and chips are rarely sold individually.
Generally, yes. From MostExpensive.com: "The world's most expensive diamond sold for $24.3 million, beating a previous record of $16.5 million for a 100-carat diamond in 1995. While the [35.56 carat] Wittelsbach is the most expensive diamond ever sold at auction -- and likely the most expensive diamond ever sold, period -- the world-famous Hope diamond [of about the same colour] would no doubt put the Wittelsbach to shame were it ever sold." Read more about this stone, below. On a more realistic level, because diamonds are so valuable, most people who own diamonds -- and other precious gems -- insure them against loss. Buying a diamond is a major purchase and often a major investment. Unlike buying a car, for example, that can end up scrapped in a junk yard over time, a diamond will always maintain its (gemologist's appraised) value.
Your question assumes that a diamond has changed hands for money. Often, the most glorious diamonds are gifts -- especially to royalty, and there is no record of any monetary value.According to Wikipedia, however:"Sotheby's just dropped the hammer on the most expensive diamond ever sold. The stone, a 59.6-carat flawless pink diamond called the "Pink Star," was auctioned for $83.2 million, according to Sotheby's. That made it the most expensive jewel or diamond ever sold at auction. Nov 13, 2013"
Diamond sizes are dependent on the size of the stone mined. Diamonds are sold by various weights, including logical increments such as quarter, half and one carat.
It is a 10 on the Moh hardness scale. This is very convenient as diamond is the definition of 10 on that particular hardness scale.
James Avery is apparently a chain retail jeweler. A 52 carat diamond is extremely large, and probably not a stone sold by this firm. You can buy a .52 (half-carat) diamond of this quality from Blue Nile today and spend from ~US$700 to ~US$800.
A 25 carat diamond will be difficult to find, since that size of stone is larger than most diamond merchants have on hand. For comparison, however, the link below shows the auction price of a 35+ carat stone, which sold for over US$24 million.