  We investigate the prospect of achieving negative permittivity and
permeability at optical frequencies in a dielectric crystal containing
stoichiometric rare-earth ions. We derive the necessary transition linewidth,
ion density and electric and magnetic oscillator strengths using a simplified
model of non-interacting dipoles. We identify Erbium crystals in a magnetically
ordered phase as the most promising material to meet these conditions, and
describe initial optical measurements of two potential candidates, \ercl{} and
${}^7$\lierf{}, which display linewidths of 3~GHz and 250~MHz, respectively.
The properties of ${}^7$\lierf{} satisfied our criterion for negative
permeability.
