Gemstones and semi-precious stones have been modified for centuries to make them more colorful, more eye-catching, and easier to work with. One such technique is heat treating (the most common) to enhance, clarify or create color in a stone. Amethyst is heated for citrine. Zircon is heated to clarify the stone to clear white. Sapphires are heated to get amazing pinks and blues. Rubies lose a purplish tint while iolite may be turned a deep blue. Heat treating can also be used to enhance the 'color change' of gems such as tanzanite. Dying of stones is also a very common practice. Agate is dyed to get pinks, purples, orange and blues. Chalcedony is also dyed; black chalcedony sold as onyx. Irradiation is another common treatment. Topaz is currently the most commonly irradiated gemstone (to get various shades and tones of blue). This is also how one obtains fabulously colored diamonds.