Spinels were originally thought to be these red gems which also fill in the blank of this title of a young adult novel by Philip Pullman, The _____ in the Smoke. What is it?

• A. Garnet
• B. Opal
• C. Emerald
• D. Ruby

Answer: (D)

Spinel and ruby share a deep red color, and for centuries jewelers often mistook red spinels for rubies because they look so similar to the naked eye. This historic mix-up even gave rise to terms like balas ruby, referring to red spinel that people once thought was ruby. Because the title in question uses this well-known misidentification, the blank is filled with Ruby. The other stones—garnet, opal, emerald—aren’t the red gems most commonly confused with spinel in gem lore, so they don’t fit the clue.