For two-qubit X-states, quantum discord D(A|B) = I(A:B) − C(A:B) measures quantum correlations beyond entanglement; it is nonzero even for separable mixed states.
Ollivier-Zurek (2001) and Henderson-Vedral (2001) introduced quantum discord D = I(A:B) − J(A:B), where J measures classical correlations via local measurements. Discord vanishes only for classically correlated states. The X-state family ρ = diag-off covers Bell states, Werner states, and separable mixed states — all visualized on the correlation plane.