Ametrine

Quartz · also called Bolivianite, Amethyst-citrine quartz, Bicolour quartz, Trystine

Ametrine is a single quartz crystal that is amethyst in one part and citrine in the other, both colours produced by iron in different states. The division follows internal growth sectors, so the boundary is a plane rather than a gradient. Effectively all natural material comes from one Bolivian deposit, the Anahi mine.

What is ametrine?

Ametrine is a single crystal of quartz that is amethyst in one part and citrine in the other, with a boundary between the two that is usually sharp enough to read as a straight line. Both halves are the same mineral, silicon dioxide, coloured by iron held in two different states.

The material is not a mixture and not an assembly. It is one crystal that grew with different faces taking up iron differently, so that purple and yellow developed side by side. Almost all natural ametrine comes from a single deposit in eastern Bolivia, which makes it one of very few gems with effectively one commercial source. It is normally cut as a rectangular step cut with the boundary running across the centre of the table, so that each colour occupies half the face-up view. The purple half is the material described in our amethyst entry.

What gives ametrine its two colours?

Iron gives both. The difference between the halves is not two different impurities but the same element in two different oxidation states and structural settings: the purple comes from an iron colour centre created by natural irradiation, the yellow from ferric iron that never formed that centre.

Quartz grows a set of rhombohedral faces that are crystallographically distinct, conventionally labelled r and z, and these sectors do not incorporate trace iron in the same way or in the same quantity. Where conditions are right the amethyst colour centre develops in one set of growth sectors and not the other, and the crystal ends up divided along the internal planes between them. This is why the boundary is planar and oriented rather than a soft gradient.

Heat destroys the amethyst colour centre and drives purple quartz towards yellow, which is the basis of the heat-treated citrine that dominates that market. Work on the Bolivian material published by GIA in Gems & Gemology established the growth-sector explanation and remains the standard reference.

How is ametrine identified?

By the ordinary quartz constants, taken on either half. Refractive index reads 1.544 to 1.553 with a birefringence of 0.009, specific gravity sits at about 2.65, and the optic character is uniaxial positive. A two-coloured stone that reads outside those figures is not ametrine, whatever it is called.

The competition is short. Bicolour fluorite is singly refractive at 1.434 and only 4 on the Mohs scale. Bicolour tourmaline reads near 1.62 with a birefringence of 0.018, high enough that facet edges visibly double through a loupe. Glass is singly refractive and usually betrays itself with rounded gas bubbles and swirl. Under crossed polarising filters ametrine shows the uniaxial interference figure of quartz and frequently the twinning lamellae typical of amethyst. The imitation that most often reaches a counter is an assembled doublet of amethyst cemented to citrine, dealt with below.

The distinguishing feature: a boundary that is a plane

The colour boundary in natural ametrine is a crystallographic surface, not a stain. Look at the stone from the side rather than through the table and the division stays flat and continuous, running through the depth of the stone at a consistent orientation, because it marks where one growth sector ended and the next began.

That is a useful observation, because the two things most often sold as ametrine fail it in different ways. A doublet has a boundary that is a cemented plane parallel to the girdle or the table, positioned where a cutter would put it rather than where a crystal would, and it usually carries a visible glue line, flattened gas bubbles trapped in the cement, or a difference in surface lustre.

Hydrothermally grown synthetic ametrine, produced in Russia since the 1990s, is harder. Its colour distribution is controlled by the seed plate, so the boundary can be planar too, and the physical constants are those of quartz because it is quartz. Separation there is laboratory work — trace-element chemistry, growth features around the seed, and infrared absorption from water-related defects that natural quartz generally does not carry.

Is ametrine treated?

Ametrine from the Bolivian deposit is generally sold as it came out of the ground, and the two colours are natural. The material that has to be watched is everything made to resemble it: partly heated amethyst, laboratory-grown ametrine, and assembled stones.

Amethyst can be heated over part of its volume to convert that part to yellow, producing a bicolour quartz that is genuine in both its colours but is not natural ametrine and should not be sold as such. The same logic applies to citrine generally, most of which is heated amethyst. None of these are defects in themselves; they are disclosure questions. Ask what a stone is, and expect a plain answer, as you would for any coloured stone we handle.

Where does ametrine come from?

The Anahi mine in the Santa Cruz department of eastern Bolivia, in the Pantanal wetlands near the Brazilian border, is the only commercially significant source. The quartz occurs in hydrothermal veins hosted in dolomitic sedimentary rock, and the deposit has supplied essentially the whole world market since it came back into organised production.

Occurrences of bicolour amethyst-citrine quartz have been reported from other countries, but none has yielded cuttable material in quantity. The practical consequence is that origin is not a value factor in ametrine the way it is in ruby or emerald: Bolivian is not a premium claim, it is the default, and a seller who presents it as a distinction is telling you nothing. The second consequence is supply, which depends on one operation and one route to market rather than on global demand.

Where the name comes from

Ametrine is a portmanteau of amethyst and citrine, coined for the material as it entered the trade in quantity in the 1970s and 1980s. It also circulates as bolivianite, after the country, and under the registered trade name trystine for certain cut goods.

The mine's own name comes from the account repeated throughout the trade of Anahi, a woman of the Ayoreo people said to have been given the deposit as a dowry on her marriage to a Spanish conquistador, the stone's two colours standing for her divided loyalties. The story is told as history in most sales material; it is more safely described as tradition. What is documented is that the source was reconnected to the international market only in the twentieth century.

What sizes are available?

Large. Rough from the Bolivian deposit comes in substantial crystals, so cut ametrine is routinely available in weights that would be exceptional in most coloured stones, with ten to fifty carat stones ordinary and larger pieces obtainable. What is genuinely awkward is the small end.

Below about five carats a stone becomes too short to divide convincingly, and the two colours crowd each other instead of reading as halves. Calibrated melee ametrine therefore barely exists as a category. Because large clean material is not scarce, the per-carat price curve is close to flat across most of the commercial range: paying a size premium for ametrine is generally paying for something the market does not charge for. Cutters exploit the abundant clean rough with concave, checkerboard and freeform fantasy cuts, and those cuts, not the weight, are where the money goes.

How much is ametrine worth?

Modestly, per carat, and almost all of the variation is in presentation rather than in rarity. The factors in order of weight are the saturation of both halves, the sharpness and placement of the boundary, the evenness of the split, clarity, and the quality of the cut.

A stone in which the purple is a real purple and the yellow a real golden yellow is worth a multiple of one where either half has drifted towards grey, because a weak half destroys the only thing the stone is for. Placement matters nearly as much: a boundary set on the centre line of a step cut, so each colour holds half the table, presents far better than one pushed to a corner. Clarity should be high, since the material is generally clean. Beyond that, judge the cut, and be sceptical of any premium claimed for origin.

Is ametrine durable enough to wear?

Yes, with one specific reservation. Quartz is 7 on the Mohs scale, has no cleavage and breaks with a conchoidal fracture, which makes ametrine suitable for daily wear including rings, and it is hard enough to resist the airborne quartz dust that dulls softer stones over years.

The reservation is heat, and it is a colour point rather than a durability one. The amethyst colour centre is destroyed by heat, so the purple half is the vulnerable half: a jeweller's torch used near an unprotected stone, or prolonged exposure to strong sunlight, can shift it towards yellow and unbalance the stone permanently. Have the stone removed before any hot work on a setting.

Warm water, mild soap and a soft brush are sufficient. Ultrasonic cleaning is tolerable for a clean stone but pointless to risk on an included one, and steam should be avoided because of the heat.

How to buy ametrine without being caught out

Look at the boundary from the side before you look at the stone face up. In a natural crystal it is a plane running through the depth of the stone at a crystallographic angle; in a doublet it is a cemented join parallel to the girdle or table, often with a glue line, flattened bubbles or a change in polish across it.

Then check both colours in daylight rather than under warm shop lighting, which flatters yellow and weakens purple. Both halves should carry real saturation, and the split should be roughly even. A refractometer reading of 1.544 to 1.553 confirms quartz and eliminates tourmaline, fluorite and glass at a stroke.

Ask directly whether the stone is natural, unheated and unassembled, and for a significant purchase ask for a laboratory report, since synthetic hydrothermal ametrine cannot be separated by eye. Finally, judge the cut hard: the material is abundant, clean and large, so the cutting is what you are actually paying for. If you want us to look for a particular size or cut, tell us what you are after.

Looking to buy ametrine? See the collection, the rough, or send us a brief.