The Short Version
The claim that at least one planet is "composed mostly of diamond" is not supported by current evidence. The best-known candidate, 55 Cancri e, has been reclassified by updated NASA models as silicate-dominated, with diamond likely comprising less than 10% of its mass. Other candidates like PSR J1719-1438 were labeled "diamond planets" in 2011-2012 headlines but lack modern confirmation of majority-diamond bulk composition. Recent Webb telescope findings show diamonds forming deep inside certain planets — but that is far from being "mostly" diamond.