The diamonds should not be so expensive but the monopoly of Dee Beers did this .
The new found mine in Kimberly in South Africa in 1800s had potential to flood the diamonds in the market which would have made the price to bring down. He managed to control the diamonds supply world wide throughout 19th century.
Dee Beers made it sure to flow the enough diamonds anually thus made an illusion that the diamonds were exceedingly rare which in turn inflated the price.
The other factors of high price are four C’s viz. Carat, Clarity, Color and Cut.
All the four C’s play equal role to decide the price of a diamond.