The 'purest' form of carbon comes from different labs. The purity depends upon the lab and how well they can purify it.
FYI, carbon can exist in various isotopes:
Carbon (₆C) [6 protons] has 15 known isotopes, from ⁸C to ²²C, of which ¹²C and ¹³C are stable.
The allotropes of carbon include graphite, one of the softest known substances, and diamond, the hardest naturally occurring substance.
Carbon bonds readily with other small atoms, including other carbon atoms, and is capable of forming multiple stable covalent bonds with suitable multivalent atoms. Carbon is known to form almost ten million different compounds.