Carbon is not commonly found in the Earth's crust in its elemental form. Instead, it is found in various compounds. Here are some of the most common forms:

* Carbonates: These are compounds containing the carbonate ion (CO₃²⁻). Examples include limestone (calcium carbonate, CaCO₃) and dolomite (calcium magnesium carbonate, CaMg(CO₃)₂).

* Organic Compounds: These include a vast array of compounds containing carbon and hydrogen, often with other elements like oxygen, nitrogen, and sulfur. This category includes fossil fuels (coal, oil, and natural gas) and biomass (living and dead organisms).

* Silicates: While carbon is not the primary element in silicates, it is present in small amounts within the complex structures of these minerals. Feldspars and micas are examples of silicates.

It's important to note that while carbon exists in the Earth's crust in these forms, it is not found in significant amounts as pure, elemental carbon. The vast majority of carbon is locked up in fossil fuels and carbonate rocks.