Light propagation in an atomic medium whose coupled energy levels form a ♦-configuration exhibits a critical dependence on the input conditions. Depending on the relative phase of the input light fields, the response of the medium can be dramatically modified and switch from opaque to semi-transparent. These different types of behaviour are caused by the formation of coherences due to interference in the atomic excitations. Alkali-earth atoms with zero nuclear spin are ideal candidates for observing these phenomena which could offer new perspectives in control techniques in quantum electronics.