This is Bolivian Ametrine, untreated and natural. The crystals form in a bi-colored way: Amethyst (purple), and Citrine (yellow). Heated Amethyst can convert to Citrine (this is how most of today's Citrine is created. Natural Citrine is a very pale yellow color compared to the heat-treated standard Citrine on today's market.) Plus, you have the chemical variations the crystals were exposed to while forming. The quality of the stone's bi-color cut is dependent on the cutter's choosing the best 50/50 to 60/40, 40/60 split and the shape.
This is the smallest Ametrine I have ever seen: the colors are there but faint. The Ametrines dominating the market today are lab-grown or have been heat/cold treated: Amethyst side cooled and Citrine side heated. Most have a very sharp line at the color change--not a found-in-nature characteristic.
Purchased from JTV ca 2003...from a personal collection / hoard
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From JTV's Gemopedia (TM):
Ametrine is a bi-color quartz variety that, as its name suggests, is a
unique combination of amethyst and citrine within a single crystal. How
the gem forms is still a bit of a mystery, but the differences in color
are believed to be the result of the presence of iron in different
states of oxidation from natural heating. Combining the golden sunburst
of citrine with the violet sunset of amethyst, this naturally colored
gem is commercially mined at a single source: the remote Anah mine in
Bolivia and is shrouded in fascinating local legends and lore.