On addition of impurities to the colourless gemstones, brilliant colours are produced. On addition of chromium to colourless corundum (Al₂O₃), a red ruby is formed. Red colour ruby is obtained when Cr³⁺ ions replace Al³⁺ ions in the octahedral sites of corundum. A green emerald emerges when chromium is added to colourless beryl (Be₃Al₂(SiO₃)₆). Green colour emerald is obtained when Cr³⁺ ions replace Al³⁺ in the octahedral site of beryl.