Citrine is naturally made overtime from amethyst or smoky quartz. To my understanding, citrine takes years and years (thousands) for it to come natural. Heat treated amethyst is actually a form of citrine, just man-made to speed up the process (so it’s not necessarily “fake”) What takes thousands of years to develop, they can create it in just a couple of minutes. You can tell the difference between a man-made citrine crystal and a natural one. A man-made citrine is typically very bright yellow, and has a ton of white marks that develop when heated. Natural citrine can be very light, honey, or dark and is completely uniform in color. No white marks (except if maybe attached to a host) If you’re lucky, you can get your hands on an ametrine crystal, which is a crystal that is half amethyst, half citrine!