Ametrine, also known as trystine or by its trade name as bolivianite, is a naturally occurring variety of quartz. It is a mixture of amethyst with zones of purple and yellow or orange. Almost all commercially available ametrine is mined in Bolivia.
The colour of the zones visible within ametrine are due to differing oxidation states. The citrine segments have oxidized iron while the amethyst segments are unoxidized. The different oxidation states occur due to there being a temperature gradient across the crystal during its formation. Artificial ametrine is created from natural citrine through beta irradiation (which creates an amethyst portion), or from an amethyst that is turned into citrine through differential heat treatment.
Ametrine in the low price segment may stem from synthetic material. Green-yellow or golden-blue ametrine does not exist naturally. This gem only exists in Bolivia, it comes from the Ricón del Tigre area, from the Anahí mine.