More than 100 tons of earth and rock must be excavated and processed to extract just one carat of diamonds. Less than half of these carats is of acceptable jewelry grade;  the rest are industrials" (diamonds too flawed for use in jewelry).

Quality is another factor of rarity. The better the grade, the rarer the stone. The vast majority of all jewelry grade gems are small diamonds used in clusters or tennis bracelets, etc.

The carat-sized diamond is fewer than one in every 25,000 carats of diamonds mined and top qualities are rarer still.

According to the GIA grading scale, D flawless is the top quality grade for diamonds. Only about 50 to 100 diamonds of D flawless quality  weighing over 1 carat are found each year.

These statistics are one reason why diamonds cost what they do.