The earth is about 47% oxygen, 28% silicon, 8% aluminum, 5% iron, and 12% other elements, such as calcium, sodium and potassium. The crust of the earth is composed mainly of feldspar, quartz and pyroxene, and the mantle (the layer beneath the crust; it makes up most of the earth's volume) is primarily made out of the mineral olivine, also known as the gem peridot. All these minerals are compounds of silicon and oxygen, with smaller amounts of other elements (for example, in olivine's case, iron or magnesium).
Below the mantle is the core, made up of two layers of an iron-nickel alloy. The top layer is liquid, while the lower layer, at the center of the earth, is solid.