Crystalline Form of Carbon Used in Pencils
Puzzle 10 of Homeschooling Pack
Question is: Crystalline Form of Carbon Used in Pencils
and the answer is: GRAPHITE
Word Meaning:
GRAPHITE — An allotrope of carbon, consisting of planes of carbon atoms arranged in hexagonal arrays with the planes stacked loosely, that is used as a dry lubricant, in "lead" pencils, and as a moderator in some nuclear reactors.
Definitions from Wiktionary.