What is the hardest metamorphic rock?

Question

I can only see with my right eye!

Answer 1

Diamond.  Natural diamonds are, in fact, metamorphic rocks.

(Try opening the left one!)

Answer 2

What does seeing with only one eye have to do with this question?

Answer 3

Well, the hardest mineral on earth is a diamond. For rocks, which are made up of one or more minerals, the hardest are those which contain quartz. There is no single "hardest rock" but some of the rocks that would be very hard and strong and well-cemented quartz sandstones (a sedimentary rock), quartzite (a metamorphic rock formed from quartz sandstone) and granites or rhyolites (both igneous rocks with a lot of quartz).