Identify the source of color in ruby

Ruby is essentially corundum, which is $Al_2O_3$ containing a small amount of $Cr^{3+}$ impurity in place of $Al^{3+}$.

Hence, the red color of ruby is due to: $Cr^{3+}$

Identify the source of color in emerald

Emerald is a variety of beryl, with formula approximately $Be_3Al_2Si_6O_{18}$, and its green color is also due to the presence of $Cr^{3+}$ ions (sometimes traces of $V^{3+}$ may also contribute, but in standard JEE context the accepted answer is $Cr^{3+}$).

Hence, the green color of emerald is due to: $Cr^{3+}$

Match with the options

• A: $Cr^{3+}$ and $Co^{3+}$ ❌
• B: $Co^{3+}$ and $Cr^{3+}$ ❌
• C: $Co^{3+}$ and $Co^{3+}$ ❌
• D: $Cr^{3+}$ and $Cr^{3+}$ ✅

Final answer

The transition metal ions responsible for color in ruby and emerald are, respectively: $Cr^{3+} \, , \, Cr^{3+}$

So the correct option is D.