Short answers, pulled from the story.
Antoine Lavoisier proved that diamonds are carbon in 1772 by burning samples of both charcoal and diamond. He found that neither produced water and that both released the same amount of carbon dioxide per gram. This experiment overturned centuries of belief that diamond was a distinct substance.
Graphite consists of layers of hexagonally arranged carbon atoms that slide easily past one another, making it soft and a good electrical conductor. Diamond features a rigid three-dimensional lattice where each atom is bonded tetrahedrally to four others, making it the hardest naturally occurring substance and an excellent electrical insulator.
Carbon forms within the interiors of giant or supergiant stars through a process known as the triple-alpha process. This process requires the nearly simultaneous collision of three helium nuclei at temperatures over 100 megakelvins and occurs in stars that have exhausted their hydrogen fuel.
The production of synthetic diamonds was first achieved in the 1950s. About 3 billion carats are produced annually for cutting, drilling, and polishing tools.
Inhalation of coal dust or soot can cause coalworker's pneumoconiosis, a congestive lung disease. Carbon monoxide is a colorless and odorless gas formed by incomplete combustion that is lethal because it binds permanently to hemoglobin, displacing oxygen.
Researchers at North Carolina State University developed Q-carbon in 2015. This new form of carbon exhibits ferromagnetism, fluorescence, and hardness superior to diamonds.