This is a picture of a Brownlee particle, which may be what a comet is made of. Click on image for full size version (40K GIF) Image courtesy of JPL

This example of Interstellar Dust is a perfect example of the kind of rocky material that may make comets. The grains themselves seem to be made of smaller grains. There are many holes, or pores. In a proper comet, these holes would be filled with ice.

This example of Interstellar Dust is called a Brownlee Particle after D. E. Brownlee. Dr. Brownlee first detected these particles in the Earth's atmosphere and thought that they may be the left over remains of comets which passed through the Earth's orbit. He came up with this theory because of the unusual construction of these rocky particles.