Just as Amethyst is a variety of quartz, so is Citrine, but more rare than Amethyst.
Traces of Iron contained in the structure of the quartz crystal cause the crystal to be either purple, yellow, orange, or green. The coloring is affected by the condition of oxidation in the crystal. Heat changes the state of oxidation. If you leave your Amethyst in the hot sun, it will change from Purple to Yellow or Orange. You will have changed your Amethyst into a Citrine. Sometimes Citrine turns into Amethyst if the crystal is near radioactive minerals. Some crystal dealers irradiate pale Citrine to make it purple. Or they heat Amyethyst to make it bright orange.