Densities and elastic properties of solid natural spinel- and garnet-peridotite samples (n = 133) at standard temperature (T) and pressure (P) (STP) conditions were calculated for compositions ranging from Mg# (100 ¿ Mg/(Mg + Fe)) of 86--94. The physical properties were used to investigate how natural compositional variations control density and seismic velocity. A corresponding set of compositional derivatives (d/dMg#) of density and seismic velocity is provided. Because the P and T derivatives of elastic moduli are very similar for different compositional end-members of peridotitic minerals, the variation of elastic moduli with Mg# at STP conditions holds at elevated P and T. Increased Mg# leads to a significant increase in VS because of the sensitivity of mineral shear moduli to this parameter (dVS/dMg# = 0.0143 ¿ 0.0009 km s-1). In contrast, the compressional wave velocity (VP) is insensitive to Mg# and, instead, correlates weakly with increasing olivine abundance at STP conditions. The ratio VP/VS therefore exhibits a significant negative correlation with Mg# (d(VP/VS)/dMg# = -0.00407 ¿ 0.00038). Because the temperature dependency of VP/VS is small (<~0.04%/100¿C) compared to the compositional dependency (1.7%/Mg# unit), the variation in VP/VS is a fairly robust measure of compositional variation even when temperature varies. Finally, a new density versus Mg# parameterization is derived. Combined with a compilation of bulk-rock Mg#s of peridotite xenoliths from cratonic lithospheric mantle, it is shown that the intrinsic density of cratonic mantle balances to within error its negative thermal buoyancy imposed by its cooler thermal state relative to upwelling asthenospheric mantle.