Possible Clues:

• "At the ___ Core" (Edgar Rice Burroughs novel)
• "At the __ Core": Burroughs novel
• Brownish pigments
• Cerium and samarium are rare ones
• "At the ___ Core" (Burroughs novel)
• "At the ___ Core"
• Of our planet
• "Core" of an Edgar Rice Burroughs title
• She has about as much craft as foxes have
• Brownish hues