Ametrine

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 and citrine with zones of purple and yellow or orange. Almost all commercially available ametrine is mined in Bolivia, although there are deposits being exploited in Brazil and India.

The colour of the zones visible within ametrine are due to differing oxidation states of iron within the crystal. The different oxidation states occur due to there being a temperature gradient across the crystal during its formation.

Artificial ametrine can be created by differential heat treatment of amethyst.

Source

 * http://en.wikipedia.org