Carbon Is Not Radioactive at all, so will not decay.
Carbon is a chemical found in most things. You can obtain carbon from coal, sugar, and anything living or that was living, you can get it from diamonds.
The chemical, cellulose, which is the product of plant-related entities: including jukeboxes, wooden planks, etc., will produce six vials of carbon when decomposed. When a spider eye is decomposed, will produce the potent chemical, Tetrodotoxin. The decomposition of tetrodtoxin will produce eleven vials of carbon. Ink sacks, derived from squids, will produce a compound known as "carbon-black".
Carbon is relatively abundant in the Minecraft universe; it behaves the same as the standard solid element. It is, to a degree, inert - but can produce a wide variety of chemicals and compounds.
More Drops From Coal And Diamond when used in a polytool