Blue diamonds are speculated to be rarer than white/clear diamonds because it is more difficult to form naturally. Blue diamonds, as such, are worth more than regular white diamonds
Every diamond is valued by its cut, colour, clarity and carat weight. A local jeweler may be able to help you compare two similar diamonds, each with a different colour, and explain to you why one...
No, they are not, They are full of unprocessed carbon that was not turned into Diamond properly, most diamonds have specs of "chocolate diamond" which most call impurities, and decrease the value...