Question

In ruby and emerald, the metal ion(s) present is/are:

• A. \(\mathrm{Cr}^{6+},\mathrm{Mn}^{3+}\)
• B. \(\mathrm{Cr}^{3+}\)
• C. \(\mathrm{Mn}^{3+},\mathrm{Co}^{3+}\)
• D. \(\mathrm{Co}^{3+}\)

Hint:

The colour of many gemstones is due to transition metal ions. Ruby and emerald both get their characteristic colours mainly due to \(\mathrm{Cr}^{3+}\).

Answer 1

The Correct Option is B

Step 1: Recall the composition of ruby.
Ruby is a gemstone consisting of aluminium oxide (Al₂O₃, corundum) with a small amount (0.5-2%) of Cr₂O₃ as an impurity. The Cr³⁺ ions replace Al³⁺ in the crystal lattice and are responsible for the characteristic deep red color.
Step 2: Recall the composition of emerald.
Emerald is a variety of beryl (Be₃Al₂Si₆O₁₈) with Cr₂O₃ as an impurity. Cr³⁺ ions replace Al³⁺ in the beryl structure and impart the characteristic deep green color.
Step 3: Identify the common metal ion.
Both ruby and emerald contain Cr³⁺ (chromium in the +3 oxidation state) as the chromophore (color-giving ion). Despite having the same Cr³⁺ ion, they appear different colors (red vs green) due to different crystal field environments.
Step 4: Explain why the colors differ despite same Cr³⁺.
The crystal field splitting energy ($\Delta_o$) depends on the ligands and geometry. In ruby (Al₂O₃), Cr³⁺ is surrounded by O^2- with one splitting; in emerald (beryl), slightly different coordination environment shifts the absorption band, changing the transmitted color.
Step 5: Confirm Cr³⁺ is the answer.
No other transition metal ion is common to both ruby and emerald. Mn²⁺, Fe³⁺, etc. are found in other minerals but not in both of these specific gemstones.
Step 6: Conclusion.
Cr³⁺ is the metal ion present in both ruby (Al₂O₃ matrix) and emerald (Be₃Al₂Si₆O₁₈ matrix).
\[ \boxed{Cr^{3+} \text{ (option 2)}} \]