شهاب سنگ

from a more complete one in Rocks From Space by O. Richard Norton. This book includes considerable explanation as well.

Iron and Stony Iron Meteorite Parents

The clumping together of the larger material, like the initial clumping, had the effect of concentrating the radio activity and resulting heat. Parts of the rock melted. The heavier material--metallic iron and nickel settled by gravity to the center of the forming mini-planet. This is the parent for iron meteorites. Outside of the iron, a mixture of iron and other minerals formed the parents of stony-iron meteorites. The process is called differentiation.

Achondrite Parents

By the theory, the melting and differentiation would also result in lighter rocks. These would be like the lava we see emerging from volcanoes on Earth. They would form the parents for achondrites. 

How Old Are Meteorites

If meteorites formed from dust from the early solar system, then we would expect them to be very old. Indeed, this is what we find. Scientists have used radiometric dating to measure the ages of meteorites. The results show ages of around 4,500,000,000 years--about seven hundred million years older than the oldest rocks on Earth.