The highest quality diamonds are found in the earth.
"Best" is a judgement, and you are the judge. The 'why' part of your question can only be answered by Mother Nature, She being the most prolific producer of excellence in the natural world. In diamond type, the most rare diamonds are of a Type II -- a and b. These diamonds are the 'purest of the pure' diamonds. Africa is not the only producer of these diamonds -- the Cullinan was found in Africa; but diamonds of this type are also found in India, i.e., the Koh-i-noor. You can read more, below.
Most gem-quality diamonds found are about the size of a vegetable, the pea. The largest diamond found to date is the Cullinan Diamond, a mighty excess of 3,000 carats: it fit into the palm of a grown man's hand. It was found in 1906. Larger gem-quality diamonds are found, but they are rare. The process of forming diamonds occurs far below the earth's surface, deep within the earth's mantle. Why the diamonds formed there are 'small' is a mystery only Mother Nature can answer.
'Value' implies a transaction for money. The largest and most spectacular diamonds in the world generally do not change hands for money; they change hands as gifts or often, spoils of war. The highest quality, gem-quality diamonds have been found in South Africa and Zaire. These include the Cullinan Diamond, and the Millenium Star, respectively. Other spectacular diamonds have been found in India. You can read more about these named diamonds, below.
Sea shells are not known as a source for diamonds.
Yes, diamonds can be found underground. In fact, most diamonds are mined from the earth.
Most diamonds are found on the earth and in the earth near indicator minerals, such as kimberlite.
Most jewelery stores in most cities in Europe sell diamonds.
You can purchase diamonds in most any city in the world. There is no 'city' where diamonds are mined, however.
Most gem-quality diamonds found are about the size of a pea.
in diamond mines
Argyle pink diamonds are the most rare and valuable and can only be found in Argyle minds.
Diamonds are found in mines, downstream from volcanic pipes that contain diamonds, and at the mouths of rivers that empty into sea water when these rivers flow over volcanic pipes that contain diamonds.
Diamonds are found in Australia, many countries in Africa, Arkansas, a US state, Brazil, Canada, India and Russia.
Canary yellow diamonds can be found ion many regions the most valuable come from South Africa.
Brown diamonds are the most common of coloured diamonds found. Chocolate is simply the name of one of the shades of natural brown diamonds. Lab-created diamonds are generally 'white' diamonds.
Diamonds are not found in a great many different places. Diamonds are not found floating in the ocean for example.