Ametrine is a combination of 2 crystalline quartz, purple amethyst and golden-yellow citrine and occurs naturally where there were temperature changes while the crystal was forming, causing the colour variation. The only commercial source of ametrine is the Anahi mine in south eastern Bolivia making it a rare gemstone.
On the left is a synthetic quartz ametrine and a block of synthetic quartz before it is cut and polished, while on the right is a natural ametrine.
But do be careful because the difference between natural and synthetic is not as simple as the colour difference between these 2 examples! Inclusions may provide indications as to whether a stone is synthetic or natural and depending how the stone has been cut, colour zoning may also provide information. A synthetic gemstone possesses the same physical, chemical and optical properties as its natural counterpart so caution must be exercised when testing these stones!