Law of mass action is not applicable to ${\mathrm{C}}_{\text{(graphite) }}\rightleftharpoons {\mathrm{C}}_{\text{(diamond) }}$ because

C graphite $\rightleftharpoons$ C diamond

for a general reaction

$\mathrm{aA}+\mathrm{bB}\rightleftharpoons \mathrm{CC}+\mathrm{d}D$

According to Law of Mass action

${\text{k}}_{\mathrm{c}}=\frac{\left[\mathrm{C}{]}^{\mathrm{c}}\right[0{]}^{\mathrm{d}}}{\left[\mathrm{A}{]}^{\mathrm{a}}\right[\mathrm{B}{]}^{\mathrm{b}}}$

Law is mass action is only applicable to chemical equilibrium. Whereas * Graphite $\rightleftharpoons$ Diamond is in physical equilibrium under certain conditions of Temperature and Pressure.