Ruby is aluminium oxide (Al2O3) in which some of the positions normal…

The correct answer is Option (1) → \( \ce{Cr^3+} \)

• Ruby is alumina containing about 0.5–1% of Cr³⁺ ions, spread at random in the places of Al³⁺. Cr (Z = 24) is [Ar] 3d⁵ 4s¹; losing three electrons gives Cr³⁺: 3d³.

• Each Cr³⁺ behaves like an octahedral chromium(III) complex inside the alumina lattice, and d–d transitions at these centres give the colour. Al³⁺ itself has no d electrons, so pure alumina is colourless.

• The same Cr³⁺ ion, at octahedral sites in the mineral beryl, gives emerald its green colour.

NCERT (2026-27 edition):
5.5.5 Colour in Coordination Compounds (Colour of Some Gem Stones) – “…Ruby … is aluminium oxide (Al₂O₃) containing about 0.5-1% Cr³⁺ ions (d³), which are randomly distributed in positions normally occupied by Al³⁺. We may view these chromium(III) species as octahedral chromium(III) complexes incorporated into the alumina lattice; d–d transitions at these centres give rise to the colour.”

“In emerald …, Cr³⁺ ions occupy octahedral sites in the mineral beryl (Be₃Al₂Si₆O₁₈).” …