Based on what was known at the time, Arthur C Clarke, with a solid science background, once suggested that at the center of Jupiter there exits a very high quality diamond about the size of earths moon. Carbon is the heaviest common element detectable in Jupiter’s atmosphere and most of it would drift to the bottom of that massive gravity well where it would be subject to immense pressure forming it into a diamond.
Nice supposition.
Once you have reliable, affordable access to space (out to about Jupiter) mineral resources are HUGELY abundant. Asteroids of solid iron, gold, silver etc are likely. Dido comets of mostly H20.