There is a known planet composed mostly of diamond.
Why it is true
The exoplanet 55 Cancri e has a carbon-rich interior which, due to extreme heat and pressure, likely exists in the form of diamond and graphite.
Source: ESA
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.