General Discussion IG6, page-223

You have that back to front, synthetic graphite is generally less combustible than natural graphite.

Synthetic graphite is produced through a high-temperature process that removes impurities, resulting in a more pure and stable form of carbon. This makes it less reactive and less prone to combustion compared to natural graphite, which may contain impurities that can contribute to its combustibility. However, it is important to note that both synthetic and natural graphite can still burn under certain conditions.