If diamonds are just lumps of precisely-structured carbon, why don't they burn? Actually, they do. And proving that they do is one of the things that cost a famous scientist his life.
I wrote last week about Antoine Lavoisier, who died, in part, because he had mocked the scientific discoveries of someone who became one of the leaders in the French revolution. One of the charges leveled against Lavoisier — just before he got his head and body forcibly separated — was his aristocratic extravagance. He had done the one thing more profligate than setting money on fire; he'd set a diamond on fire. The fact that he had done it to prove that diamonds are carbon, and that matter is conserved in chemical reactions, carried no weight with the leaders of the revolution, but it does have some significance to us.