Amethyst

Quartz · also called Purple quartz

Amethyst is the purple variety of quartz, coloured by iron in combination with natural irradiation. It is the most abundant of all gem varieties in fine quality, which is why it costs a fraction of what its appearance suggests, and it is the only common gemstone whose colour can be destroyed by sunlight.

What is amethyst?

Amethyst is quartz — silicon dioxide — coloured purple by iron that has been activated by natural radiation in the ground. It is the most valuable of the common quartz varieties and, in fine colour, one of the most beautiful stones available at a low price.

That last point is the whole commercial story. Amethyst of a quality that would cost thousands per carat in a scarce species costs a few dollars, because Brazil and Uruguay alone produce it in quantities the market cannot absorb. Nothing about its beauty is diminished by that; only its price is.

What gives amethyst its colour?

Iron substitutes for silicon in the quartz structure, and natural gamma radiation in the host rock then knocks an electron loose to form a colour centre. The purple is therefore not simply iron: it is iron plus a defect, and that distinction explains everything unusual about the stone.

Because the colour depends on a defect rather than on a stable ion, it can be undone. Heat destroys it. Prolonged strong light destroys it slowly. And irradiation can put it back — which is why both heating and irradiation are used commercially on this material.

How is amethyst identified?

Amethyst reads 1.544 to 1.553 with a birefringence of 0.009 and a specific gravity of 2.65 — the standard quartz figures, and low enough to separate it immediately from purple sapphire at 1.76, from spinel at 1.718, and from purple garnet at 1.74 and above.

The more useful field feature is colour zoning. Natural amethyst very often shows angular bands or blocks of stronger and weaker colour, following the growth of the crystal. Held to a light and rotated, most natural stones betray this structure somewhere.

The difficulty is synthetic amethyst, which has been grown hydrothermally in quantity since the 1970s and can be very hard to separate — sometimes requiring advanced testing. For low-value material nobody bothers, which is a commercial reality worth stating plainly rather than pretending otherwise.

Why does amethyst fade?

Because its colour centres are not permanently stable. Prolonged exposure to strong sunlight will bleach amethyst toward grey or colourless over years, and heat will do it in minutes.

This is not a defect in poor material — it is a property of the species. Store amethyst away from windows, do not leave it on a sunny sill, and never expose it to a jeweller's torch. Once faded, the colour can be restored only by irradiation, which is a laboratory process rather than something a jeweller can undo.

Is amethyst treated?

Frequently, and in ways that produce entirely different stones.

Heating at moderate temperature turns most amethyst yellow or orange — which is where the great majority of commercial citrine comes from. Heated at different temperatures, certain Brazilian material turns green, and that green stone is sold as prasiolite. Both are legitimate products; both should be disclosed, and both are routinely not.

Irradiation is used to deepen weak colour or to restore faded material. It is stable enough in practice but is another thing that ought to appear on a description.

The honest position on citrine is worth stating: natural untreated citrine is genuinely uncommon, and most of what is sold under that name is heated amethyst.

Where does amethyst come from?

Brazil dominates, chiefly Rio Grande do Sul and Bahia, where basalt geodes produce enormous quantities. Uruguay, around Artigas, yields the deepest and most saturated colour and commands a premium for it, often with a reddish rather than a bluish tone.

Zambia, at Kariba, produces material with a distinct bluish-violet cast that many dealers rate above Brazilian. Bolivia, at the Anahí mine, is the source of ametrine — amethyst and citrine zoned in a single crystal. Russia, Madagascar, India and Mexico contribute smaller quantities, and the historic Ural material still trades under the old grading term Siberian.

Where the name comes from

From the Greek *amethystos*, meaning not intoxicated. The ancients believed a cup carved from the stone, or a stone worn while drinking, would prevent drunkenness — an association that lasted long enough to make amethyst a bishop's stone in the Christian church.

Until the eighteenth century it was counted among the cardinal gems alongside diamond, ruby, sapphire and emerald, and priced accordingly. The Brazilian discoveries of the 1700s and after ended that: supply arrived in a quantity no cardinal gem could survive, and amethyst fell to the position it holds today.

What sizes are available?

Almost any. Clean amethyst of twenty, fifty or a hundred carats is obtainable without difficulty, and large carved pieces are a normal part of the trade rather than a curiosity.

The consequence for pricing is unusual: size adds much less to the per-carat price here than in scarce species. A fifty-carat fine amethyst does not cost fifty times a one-carat one. What you pay for instead is colour, and above all evenness of colour across a large stone, which becomes harder to find as size increases.

How much is amethyst worth?

Colour first, and by a wide margin. The most valued material is a deep, even purple with red or blue flashes, without the greyish cast that spoils ordinary goods and without windows of paleness. Uruguayan and Zambian material sits at the top; ordinary Brazilian at the bottom.

Then evenness — the zoning that helps identification hurts value, because a stone with a pale end reads as a pale stone. Then clarity, which should be eye-clean at these prices, and then cut.

Prices remain in the low tens of dollars per carat even for good material, and only exceptional large stones of top colour reach three figures. Treat any amethyst offered at gem-scarcity pricing as mispriced.

Is amethyst durable enough to wear?

Yes, with one reservation. At 7 on the Mohs scale and with no cleavage, amethyst resists both scratching and chipping well enough for daily wear, and it takes an excellent polish.

The reservation is light and heat, not hardness. Keep it out of prolonged direct sun, keep it away from any flame, and be careful with ultrasonic cleaners on stones with visible fractures. Warm water and mild soap are all it needs.

How to buy amethyst without being caught out

Rotate the stone under a light before agreeing anything. Zoning is the single most common defect and it is easy to hide by photographing from the strongest angle. A stone that is superb face-up and pale from the side is a stone cut to flatter the camera.

Ask whether it has been irradiated, and be relaxed about heating on citrine and prasiolite provided it is disclosed. And be suspicious of price: at these levels there is no reason for a seller to be evasive, so evasiveness itself suggests the stone is not what it is called.

See what we hold in gems and specimens, or tell us what you are looking for.

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