Which of the following treatments is often used to enhance the color of kunzite?

Kunzite, a variety of spodumene, is known for its beautiful pink to lilac colors that can vary in intensity. The correct treatment used to enhance the color of kunzite is natural irradiation. Natural irradiation occurs when minerals are subjected to natural radiation over long periods, which can lead to a change in color. In the case of kunzite, this process can intensify its naturally occurring hues.

Other treatments, such as heating, are not typically associated with kunzite color enhancement. While heating can enhance or modify the color of various gemstones, kunzite is not usually subjected to this treatment, as it can lead to undesirable changes or even damage due to the stone's sensitivity to heat.

Natural coloration and 'none of the above' are also not applicable in this context, as kunzite does not undergo any additional natural processes that would enhance its color beyond what is derived from natural irradiation. Thus, the application of natural irradiation is recognized as a common and effective method for enhancing the color in kunzite gemstones.