Citrine is a form of yellow or orange quartz colored by iron oxides within the crystal structure. Most citrine on the market is heated amethyst or smokey quartz. Most popular gem after amethyst. 

Can co-occur with Amethyst creating Ametrine.

Natural citrine tends to be lighter in color except famous deposits like deep orange  Maderia. Washington state is known to produce some natural citrine within geode nodules.

Natural citrine can sometimes have a smokey hue. 

Citrine is the birthstone of  and is known for promoting wealth.