Rubies and sapphires are varieties of the mineral corundum (Al2O3). The red colour of the ruby comes from trace amounts of chromium. There are many beautifully coloured sapphires. Blue is the most commonly used sapphire in jewellery. Trace amounts of titanium and iron give the sapphire a blue colour. The corundum stone in this necklace fades from blue at the bottom to red at the top.