Explanation:
Ruby is a variety of corundum (α-Al2O3) that contains chromium (Cr3+) ions as impurities. The red color of ruby is due to the d-d transition of Cr3+ ions in Al2O3 lattice.

D-D Transition:
The d-d transition involves the excitation of an electron from one d-orbital to another d-orbital of the same metal ion. When a photon of light is absorbed, it excites an electron from the lower energy d-orbital to a higher energy d-orbital. The energy gap between the two d-orbitals corresponds to the energy of the absorbed photon. The color of the compound is determined by the energy of the absorbed photon.

Chromium in Ruby:
In ruby, chromium ions replace some of the aluminum ions in the corundum lattice. The chromium ion has a partially filled 3d orbital, which can undergo d-d transitions. When a photon of light is absorbed by a chromium ion, an electron is excited from the ground state (t2g) to the higher energy state (eg). The energy required for this transition corresponds to the red part of the visible spectrum.

Al2O3 Lattice:
The Al2O3 lattice has a distorted octahedral geometry due to the Jahn-Teller effect. The chromium ion is located at the center of the octahedron and is surrounded by six oxygen atoms. The oxygen atoms form a distorted octahedron around the chromium ion. The distortion of the octahedron causes a splitting of the d-orbitals into two different energy levels, t2g and eg. The t2g orbitals are lower in energy and are occupied by three electrons, while the eg orbitals are higher in energy and are unoccupied.

Conclusion:
The red color of ruby is due to the d-d transition of Cr3+ ions in Al2O3 lattice. When a photon of light is absorbed by a chromium ion, an electron is excited from the ground state (t2g) to the higher energy state (eg). The energy required for this transition corresponds to the red part of the visible spectrum.