Hard Mineral That Creates Sapphires and Rubies
Puzzle 5 of Ghost Hunting Hard Pack
Question is: Hard Mineral That Creates Sapphires and Rubies
and the answer is: CORUNDUM
Word Meaning:
CORUNDUM — (mineralogy) An extremely hard mineral, a form of aluminum oxide with the chemical formula Al₂O₃, that occurs in the form of the gemstones sapphire and ruby; it is used as an abrasive.
Definitions from Wiktionary.