Try Harrods
It is currently sold at London Drugs (Canada).
Commercial diamond is diamond sold for money.
An area where diamond gemstones are bought and sold.
The "Crown of Light" diamond, is a diamond only sold at Diamond International. It is very beautiful.
The Wittelsbach, a 35.56-carat blue diamond mined in India, sold December 2008 at a Christie's auction for $24.3 million.
Blue diamonds are extremely rare, the price for is usually determined by the auctions. For example The Wittelsbach Diamond was sold for record $24.3 million at auction in London in 2008. See the link below for more information on blue diamonds and their prices.
It is not possible to find out the rarity of a diamond that is sold online. The only way to figure it out, would be to purchase the diamond and then have a professional look at the diamond.
Address of Diamond Continental Bank in London
Margaret Diamond was born in 1916, in London, England, UK.
Hatton Garden, Camden town, London
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.)
Any such visible flaw would be removed in the diamond cutting process, if the diamond is to be sold as a gemstone.