Citrine has a trigonal crystal system and is in the mineral class of oxide in the quartz group. It gets its yellow color from small amounts of iron and slightly ionizing radiation. In contrast to Amethyst, the iron in Citrine has been changed by exposure to some sort of super-heating changing its chemical structure from Fe2 to Fe3 (basically burning the iron).
Citrine is formed in the same primary manner as Amethyst and is found in pegmatites or formed hydrothermally in druses or clefts. Some of the Citrine in the marketplace today is almost brown looking. This is due to the fact that its probably Amethyst that has been “baked”. While I have found it to hold many of the properties of naturally occurring Citrine, it does not seem to be as potent as when Mother Earth has done the cooking.