Citrine geodes are naturally occurring crystal formations that have a pronounced yellow or orange tint. The crystals inside the geode cavity are usually a form of amethyst that has been subjected to high temperatures by, for example, exposure to lava.
A geode begins as a bubble in rock. Over time, mineral-rich water seeps through the matrix surrounding the interior cavity and deposits its dissolved minerals as crystals. The mineral components of the water determine the properties of the subsequent crystals, with traces of magnesium imparting a rich purple color. If, after the crystals are deposited, the geode is heated, the purple crystals are likely to turn citrine orange.