The incorrect statement among the following is:

CONCEPT:

Coloration in Gemstones

• The colour of gemstones, such as rubies, is due to the presence of certain transition metal ions in their crystal structure.
• These transition metals can have different oxidation states, which contribute to the color formation through the absorption of specific wavelengths of light.
• Common examples include Cr³⁺ in rubies (red), and Fe²⁺ and Fe³⁺ in sapphires (blue).

EXPLANATION:

• Rubies are primarily composed of aluminium oxide (Al₂O₃) and get their red color from the presence of chromium ions (Cr³⁺).
• It is the Cr³⁺ ions that replace some of the Al³⁺ ions in the crystal lattice of the aluminum oxide, imparting the red color.
• CrO₃ refers to chromium trioxide, which is a different compound and does not contribute to the color of rubies.
• CO₃^2- refers to carbonate ion, which also does not relate to the coloration of rubies.

Therefore, the correct statement is The red color of ruby is due to the presence of Co³⁺.