Subject: Hardness of rocks

We are a class of fifth graders, studying rocks.
What is the hardest rock on Earth?
How many types and subtypes of rocks have been classified
    for planet Earth?
What is the most expensive rock in the world?
What is the largest diamond in the world? What is its size
    and what is it called? Where is it?
Where is the biggest golden nugget located?

Thank you for your attention to our questions.

Re:Hardness of rocks