Which two gemstones are varieties of the same mineral?

Sapphire and ruby are gem varieties of the same mineral, corundum. They crystallise in the trigonal system. Although both are aluminium oxide, the addition of different trace elements gives the two varieties different colours. All colours of gem corundum except red are called sapphire. Red corundum is called ruby. Ruby owes its colour to traces of chromium in its structure, while sapphires derive their colour from trace elements such as iron, titanium and nickel.

Source: Australian Museum