What primary materials constitute the bulk of a terrestrial planet?

Silicates and metals (iron and nickel)

The fundamental construction of a terrestrial planet is centered around dense, refractory materials. Specifically, the majority of its mass is composed of silicates, which form the rock constituting the mantle and crust, and various metals, principally iron and nickel, which dominate the dense core. This high proportion of heavy, high-melting-point elements results in the characteristically high density observed in these worlds when contrasted with gaseous or icy planets. The process of condensation in the hot inner regions of the early Solar System favored the solidification and accumulation of these refractory substances over lighter, more volatile compounds.