Monazite is a reddish-brown phosphate mineral containing rare earth metals. It occurs usually in small isolated crystals. There are actually at least four different kinds of monazite, depending on relative elemental composition of the mineral:

The elements in parentheses are listed in the order in which they are in relative proportion within the mineral, so that lanthanum is the most common rare earth in monazite-La, and so forth. Silica, SiO₂, will be present in trace amounts, as well as small amounts of uranium and thorium. Due to the alpha decay of thorium and uranium, monazite contains a significant amount of helium, which can be extracted by heating.

Monazite is an important ore for thorium, lanthanum, and cerium. It is often found in placer deposits. The deposits in India are particularly rich in monazite. It has a hardness of 5.0 to 5.5 and is relatively dense, about 4.6 to 5.7 g/cm³.

Because of the presence of thorium within monazite, it can be radioactive. If samples are kept, they should be placed away from minerals that can be damaged by radiation. Because of its radioactive nature, the monazite within rocks is a useful tool for dating geological events, such as heating or deformation of the rock.