What are asteroids made of?

Asteroids are made of rock and metal and broken away pieces of planets. They are grouped into three categories: Stony, Iron-Nickel, and a mixture of the two. Most asteroids that we know about fall into the first category, and are made of Silicates. A small percentage are Iron-Nickel. The rest form the third type. I believe that they are made of rocky bits left over from the formation of the inner planets.

The run-of-the-mill asteroid is just one kilometer long. This asteroid would have a mass of two billion tonnes, of which 200 million tonnes would be iron, 30 million tonnes would be high-quality nickel, 1.5 million tonnes would be the strategic metal cobalt, and 7,500 tonnes would be a mixture of the platinum group metals whose average values at 1996 prices would be in the neighborhood of $20,000 per kilogram. That adds up to $150 billion for the platinum alone.

additional. Comets on the other hand are mainly made of frozen gases such as water, carbon dioxide, and methane. They may have collected a small quantity of dust as well.

There may well be no well accepted difference between the asteroids and comets.

[The Sudbury Crater in Canada was an asteroid that landed, and has been smeared out by geological movement. It is an important nickel deposit. ]

in the asteroid belt the asteroids contain either rock or metal but sometimes asteroids contain both rock and metal.
There's no one answer; it will depend on what atoms coalesced to form it. In general, we can expect predominantly silicate rocks with some metals; iron, magnesium, and the like. Based on the impact residues of asteroids which have struck the Earth, we can expect a higher-than-average proportion of iridium.