There's diamonds, and then there's diamonds:

The heart of that burned-out star with the no-nonsense name [BPM 37093] is a sparkling diamond that weighs a staggering 10 billion trillion trillion carats.

It's been hypothesised that Jupiter and perhaps Saturn have gigantic spheres of diamond inside them at the level where pressure and heat would create such things. Arthur C. Clarke included the idea in his "2001" series (2061), I'm sure it's been used elsewhere.

At that density and size, it should be noted, the thing has a gravitational field about three thousand times stronger than the earth's.