Here's why:

* Carbon exists in many forms (allotropes): The most common forms are diamond and graphite.

* Diamond: Extremely hard due to its strong covalent bonds, making it solid at room temperature.

* Graphite: Soft and flaky due to layered structure, still solid at room temperature.

While carbon can exist in other forms (like fullerenes or nanotubes), these are also solid at room temperature.