How the gem stones ruby exhibits red colour and emerald exhibits green colour ?

Explanation

Ruby is aluminium oxide (`Al_2O_3`) containing about 0.5-1% `Cr^(3+) "ion"(d^3)`. Thus chromium (III) is an octehedral complex incorporated into the alumina lattice. The d-d transition in `Cr^(3+)` give the red alumina lattice . The d-d transition in `Cr^(3+)` give the red colurs to ruby . Emerald is the beryl meineral `(3BeO. Al_2O_3. 6SiO_2)` having some `Cr^(3+)` ions at octehedral sites. The absorption bands seen in the ruby shift to longer wavelength, namely yellow-red and blue, causing emerald to reflect green colour.