Compiled laboratory measurements of the densities (p) and compressional (V•) and shear (V s) wave ve-locities of common rocks demonstrate that the velocities of silicate rocks increase with density along the Nafe-Drake curve as the rocks become more mafic and increase in metamorphic grade. Reflection coefficients calculated from impedance contrasts between felsic and mafic rocks and other common lithologies in hardrock terranes often exceed 6%, the estimated minimum value required to give strong reflections. This finding is consistent with observations of deep structure and stratigraphy in high-grade terranes using seismic reflection. Sulfides lie far to the right of the Nafe-Drake curve in a large velocity-density field controlled by mixing lines connecting individual sulfide minerals and their hosts. The elastic properties and acoustic impedances of massive sulfides differ significantly from those of most common host rocks, suggesting that large massive sulfide deposits should be detectable as reflectors or scatterers using high-resolution seismic reflection techniques, a prediction confirmed by recent VSP, 2D and 3D MCS surveys in Canada. Laboratory measurements of acoustic velocities under in-situ conditions provide the basis for the interpretation of seismic refraction and stacking velocities in terms of lithology.