Mystery #3Tier 2rocks under pressure
How can a soft, crumbly rock slowly turn into hard, sparkling stone?
Imagine a lump of soft, crumbly rock you could scratch with your thumbnail. Now imagine hard, glittering stone that a chisel can barely mark. Amazingly, the first can slowly turn into the second, all by itself. How can one kind of rock become a completely different one?
Your guess
What do you reckon? How could one kind of rock become a totally different one?
There's no single magic word to type, have a go in your own words, then reveal the answer to see how close you got.
Make a guess to unlock the hints and the answer.