Your planet's carbon (20.0%) outweighs its oxygen (12.5%), so the mineralogy flips from silicate rock to carbide - a graphite crust over a diamond mantle.
Carbon worlds form in carbon-rich disks where there isn't enough oxygen to bind it all into silicates. Instead carbon condenses directly into graphite, carbides, and - under pressure at depth - diamond. They form water-poor, so no life takes hold.
Matched on physical properties only — but pulsar planets are real: the first exoplanets ever confirmed (PSR B1257+12, 1992) orbit a neutron star.Discovered 2012 via radial velocity · 42 light-years awaySource: Extrasolar Planets Encyclopaedia