No, comets are not mostly made up of rock and metal. While they do contain some rocky and metallic material, they are primarily composed of ice (frozen water, carbon dioxide, methane, and ammonia) and dust.

Here's a breakdown of the typical composition:

* Ice (70-80%): This is the dominant component, giving comets their characteristic "icy" nature.

* Dust (10-20%): This is a mixture of silicates, carbon-based molecules, and other compounds.

* Rock and Metal (less than 10%): These are present in smaller amounts.

So, while comets have some rocky and metallic material, their main constituents are ice and dust. This is why they are often described as "dirty snowballs" or "icy dirtballs."