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Which transition-metal ions are responsible for the colours of ruby and emerald, respectively?

1. Cr3+ and Cr3+
2. Cr3+ and Co3+
3. Co3+ and Cr3+
4. Co3+ and Co3+

Correct answer: Cr3+ and Cr3+

Solution

Both ruby and emerald owe their colour to trace Cr3+ ions; the differing host lattice changes the crystal-field environment, giving red versus green.

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