Carbon is often seen in lung sections, particularly from people who live in urban and industrialised areas. Smoke, smog and exhaust all contain carbon particles which are breathed in and stay in the lungs. These deposits appear as black particles, sometimes as finely divided particles, sometimes as quite heavy clumps. Carbon is unreactive, and is proven by exclusion, since all other black deposits can be demonstrated or removed. If a dark granular deposit is not stained by a selective staining method, then treat sections with strong bleaches and strong acids and all granular deposits but carbon will be removed.
Also, of course, there is the minor matter of it being the basis for all organic chemistry.
Chemistry: Structure and Changes of Matter
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