The refractive index of a medium depends on the electric and magnetic properties of the medium and as a result it is very unique is dependent on the way the medium responds to electric and magnetic fields.
The velocity of light in a medium,
#v = 1/sqrt(muepsilon)# where #mu# and #epsilon# are the magnetic permeability and electric permittivity of the medium respectively.
Also, velocity of light in vacuum,
#c = 1/sqrt(mu_0epsilon_0)# where #epsilon_0# and #mu_0# are the permittivity and permeability of free space respectively.
Therefore, the refractive index,
#n = c/v = sqrt ((epsilonmu)/(epsilon_0mu_0))#
But, #epsilon_0# and #mu_0# are fundamental constants and hence #n# depends on electric and magnetic properties #epsilon# and #mu# of the medium which are unique of the medium.