Diamonds are eternal and never degrade.
Why it is true
Contrary to popular belief, diamonds are not eternal. They are only metastable and can turn into graphite over billions of years under certain conditions. They can also burn at temperatures above 700 degrees Celsius.
Source: Nature
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.