How are many green and blue tourmalines lightened?

Heating is a recognized method for lightening many green and blue tourmalines. This process involves subjecting the stones to elevated temperatures which can enhance their color and clarity, making them more visually appealing. Heat treatment can alter the chemical composition of the minerals within the tourmaline, bringing out brighter tones and removing unwanted color zoning or inclusions.

This technique is widely used in the gemstone industry to improve the marketability of these stones. It's important to note that heating is a common and accepted practice, as the treatment is typically stable and does not pose any significant risks to the stone's integrity.

Other methods such as exposure to sunlight, chemical treatment, and natural erosion do not effectively correspond with the process typically employed for improving the appearance of tourmalines in the same way that heating does, making it the most suitable answer in this context.