Question

diamond and coal....both are made up of carbon so why diamond is too expensive and carbon is cheap

Answer 1

So, which would you pay the most for: A nice, steaming, melty-cheese pizza with a pitcher of ice cold beer, or a bucket that contains the pizza and beer all blended together into a cool, gooey, uniform orangey paste? You have to return both the bucket and the pitcher, so they have exactly the same ingredients.

Form matters, and diamond’s carbon is in a different form than coal’s.

What would you rather buy: A meal in a world where you have literally eaten that same meal every day of your life, or something delicious and different?

Scarcity also matters, and there’s a lot more accessible coal on this world then there are accessible diamonds.

(p.s. Can you tell I skipped lunch today?)

Answer 2

Because it not fulfil all the property of carbon.
And this is very hard , brighter, and found in free state that's why.