Here's how to figure out which element is NOT among the eight most common in Earth's crust:

* The Eight Most Common Elements: Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium, Magnesium.

Now, let's consider some common elements and see if they fit:

* Carbon: While essential for life and abundant in the Earth's interior, it's not one of the top eight in the crust.

Therefore, Carbon is not one of the eight most common elements in the Earth's crust.