= Solution
\b[<Graphite>:] each <carbon> forms three strong in-plane <sigma bonds> in an sp$^2$ honeycomb layer. The remaining p orbitals form delocalized pi bands, allowing electrical conduction especially along the layers. Much weaker interlayer attraction permits sliding, explaining softness without implying weak in-plane C-C bonds.
\b[Carborundum is <silicon> carbide, SiC.] A suitable structure is a three-dimensional, tetrahedrally coordinated covalent network: each Si has four C neighbors and each C four Si neighbors. Strong directional bonding in all directions resists slip and gives hardness. Its filled bonding states are separated from empty states by a substantial <band gap>, giving low conductivity when pure and undoped.
“Non-conductor” is the intended contrast with <graphite>; SiC is more precisely a <wide-bandgap semiconductor>, whose conductivity can change with doping and temperature.
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