Origins of Ametrine, Part 1: A Natural Bi-Colour Quartz
By Yaĝé Enigmus
A wonderful and less commonly known variety of quartz which exhibits zones of amethyst alongside zones that display colours often associated with citrine, ametrine is a gorgeous bi-coloured quartz named in reference to both amethyst and citrine. The striking contrast between this stone’s distinct colours make ametrine a lovely gemstone that stands out among the solid-coloured stones which are encountered in most jewelry. Some people are caught off guard by the unusual appearance of this gemstone and may wonder whether or not this quartz marvel came into being through natural processes; even when ametrine was first introduced to the contemporary gemstone market in the 1970’s, there were many who questioned the authenticity of the material, thinking its colour zoning must have been the result of treatments applied to quartz by humans. In fact, ametrine is a very real and completely natural wonder of the mineral realm.
A large emerald cut ametrine from Bolivia. Image: Skyjems
There are a number of bi-coloured and polychromatic stones seen on the market today, but what makes ametrine unique is that its colour variation is the result of changes in ambient temperature conditions during formation rather than changes in trace element content, as is the case with many other multi-coloured stones. The physics and chemistry of ametrine’s formation very closely resemble the molecular mechanisms behind the process of converting amethyst into simulated citrine via heat treatment; when amethyst is heated to the correct temperature, the irradiated quadrivalent iron (Fe4+) involved in amethyst’s FeO4 colour centre is destabilized and converts back to ferric iron (Fe3+) causing the colour of the stone to shift from violets and purples to golds and oranges. The same chemical process occurs naturally during the formation of ametrine, but only some portions of the stone’s amethyst colour centres are converted due to a temperature gradient in the crystals’ host environment and/or uneven changes to the host environment’s ambient temperature over time. Typically the converted amethyst colour zones form under the negative rhombohedral faces (z-faces) of an individual ametrine crystal, and the unconverted amethyst zones form under the positive rhombohedral faces (r-faces), with this arrangement famously producing a “pinwheel” or “trapiche” pattern if the crystal is cut perpendicular to its c-axis.
An ametrine crystal sliced perpendicular to its c-axis, revealing a pattern resembling a pinwheel or “trapiche”; Image: Mindat
What is important to note about the golden zones of ametrine, is that the ferric iron present is still intimately associated with oxygen and consequently the colour centres it is involved in are regarded as components of finely dispersed iron oxide mineral inclusions rather than a structural impurity of the quartz itself, and this does not meet the mineralogical definition of citrine. This is also true for other forms of amethyst which display a golden or orange colour as a result of heat exposure, whether the change of colour occurred due to geothermal processes or heat treatments employed by humans. Currently there is no clear consensus on what the exact trace elements are that produce citrine’s colour, with ionic iron and irradiated aluminum being the top contenders, but quartz which exhibits a golden, yellow, or orange colour due to inclusions of iron oxide minerals is unquestionably defined as a form of “ferruginous quartz” rather than true citrine. Despite this, ametrine retains its name as an affectionate nod to the similarities in appearance between this type of ferruginous quartz and genuine citrine.
Various coloured quartz crystals shown before a round of heat treatment (Top) and after treatment (Bottom) to illustrate the distinct physical/chemical properties of each variety. (Left) a crystal of amethyst which has already been heat treated once to change its colour, (Centre) two naturally occurring citrine crystals, (Right) two amethyst crystals. Image: Mindat
After its contemporary introduction, ametrine was thought by some to have been produced by heating only certain portions of amethyst crystals, but it is not possible to do this and achieve the sharp contrasting edges between ametrine’s colour zones due to the conduction of thermal energy that would occur in a diffuse manner across the crystal. In 1981, methods for producing ametrine were developed which involved fully heating amethyst crystals and then re-exposing limited portions of the stones to beams of human-generated radiation. This would revert these sections back to colours associated with amethyst, and was a large supporting factor behind the scepticism toward claims that ametrine occurred naturally. In a contemporary context the specific hues seen in these treated ametrines are most similar to some of the less desirable and less common colour schemes seen in naturally occurring ametrine, making it possible for such stones to stand out among untreated gems. Methods for producing bi-coloured hydrothermally grown quartz have also been able to replicate the colour schemes of natural ametrine, but the saturation, near perfect colour distribution, and typically flawless nature of these stones also makes them somewhat easy to spot when compared to genuine ametrine. Despite the incredible ingenuity that has gone into reproducing the alluring two-toned quartz, the true beauty of high-quality, genuine ametrine is a natural phenomenon that can only be made real by the Earth itself.
Concave facets contribute to the striking look of this 27.5 ct ametrine. Image: Robert Weldon/GIA, courtesy Minerales y Metales del Oriente.
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© Yaĝé Enigmus