Asteroids are made mostly of rock and metal, with their composition varying depending on their location in the solar system. The main types of asteroids are:

• C-type (carbonaceous) asteroids: These are the most common, comprising about 75% of asteroids. They are dark and made mostly of clay and silicate rocks containing organic carbon and sometimes water ice. They are found mostly in the outer regions of the asteroid belt.
• S-type (silicaceous or stony) asteroids: These are made mainly of silicate minerals and nickel-iron metal. They are found mostly in the inner asteroid belt closer to Mars.
• M-type (metallic) asteroids: These asteroids consist primarily of metallic iron and nickel and are thought to be remnants of the metallic cores of planetesimals. They lie mainly in the middle of the asteroid belt.

Some asteroids may also have experienced heating and partial melting, causing differentiation, such as the asteroid Vesta which has a basaltic surface. Additionally, smaller or rubble-pile asteroids can be more loosely held together by gravity and may contain other elements like nitrogen, hydrogen, oxygen, and precious metals in smaller amounts.