Amethyst

Amethyst is the violet variety of quartz and, with a Mohs hardness of 7, one of the hardest gemstones of all. It owes its color to incorporated iron, which only turns violet through the natural radioactivity of the surrounding rock, a process that takes thousands of years. Unlike the microcrystalline quartz varieties agate and jasper, it usually forms well developed individual crystals, packed tightly in druses and geodes.

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How amethyst formed

At its core it works as with agate: a cavity in volcanic rock, water containing silica, and above all time. What makes amethyst violet instead of banded is an additional ingredient: iron in the crystal lattice and the natural radioactivity of the surrounding rock.

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A cavity forms in the lava rock

When lava reaches the surface or forces its way into fissures underground, dissolved gas escapes as bubbles. As the rock cools from the outside, some of these bubbles remain as empty cavities, known in geology as amygdales or druses. The large amethyst deposits of Brazil and Uruguay go back to huge volcanic eruptions around 130 million years ago, which covered today's border region between the two countries with basalt flows kilometers thick.

Water brings dissolved silica

For a long time, hot hydrothermal water above 100 degrees was considered the cause of crystal growth. Newer studies of the geodes of Artigas in Uruguay, however, show that quartz formation there began at surprisingly low temperatures of 15 to 60 degrees: ordinary groundwater saturated with silica that seeped into the cavities through fine cracks over very long periods of time.

Quartz grows and turns violet

At first the quartz grows colorless. Only when iron ions are built into the crystal lattice and the natural radioactivity of the surrounding rock changes their charge over thousands of years do so-called color centers form, which make the light appear violet. Because this change acts from the outside, usually only the outer crystal tips, the last to grow, are strongly colored, while the base at the edge of the cavity stays pale, exactly the zoning that shows in every amethyst that has been sawn open.

The druse is fully grown

If this process continues undisturbed, the cavity becomes lined with crystals pointing inwards from all sides. If you saw such a druse open, the cut shows the agate bands on the outside, then a dense row of amethyst crystals with pale bases, V-shaped growth zones and violet tips, often crossed by fine red films of iron oxide. This is how the cathedral geodes several meters high, for which Brazil and Uruguay are famous, come into being, as well as the much smaller druses and vein fillings found in the Black Forest.

How to recognize it and what it is confused with

The most reliable feature is the violet color itself, together with a vitreous luster and the crystal shape typical of quartz: six-sided prisms with a pyramidal tip. A cut often shows the characteristic zoning, pale at the base and darker towards the tip, sometimes in soft gradients, sometimes in clearly separated sectors.

It is most easily confused with colored glass or with fluorite, which also occurs in violet but, with a Mohs hardness of only 4, is much softer and shows a distinct cleavage that amethyst lacks. One special point concerns its own family: if amethyst is heated to about 350 degrees, it first loses its color and then turns yellow, becoming citrine. By far the largest part of the citrine on the market was made this way, natural yellow quartz is much rarer.

Where it occurs, with a focus on Germany

Today the most important deposits are in Brazil, above all in Rio Grande do Sul, and in neighboring Uruguay, along with Zambia and Namibia as important African producers. Historically, amethyst was already mined in ancient Egypt on the Red Sea, as well as in India and on Ceylon.

In Germany amethyst has been recorded in almost every federal state, particularly often in Saxony and Bavaria. Closer to Auggen there is an occurrence in the southern Black Forest: near Sulzburg in the Münstertal valley, just a few kilometers away, hydrothermal veins of baryte and quartz run through the rock. From the 15th to the 19th century the area was a mining district, first for silver, later mainly for antimony. In the druse cavities of these veins, pale amethyst sits alongside agate, fluorite and fine hematite, often all three together in a single piece. Today the old vein outcrops are only accessible in places; a storm exposed some of them again in 2005, but most spots are now heavily overgrown.

From antiquity to mass-market stone

The name amethyst comes from the Greek amethystos, not drunken, because the Greeks and Romans believed the stone protected against the effects of wine. In Egypt it was already worked into amulets and scarabs in the fourth millennium BC, and the Roman scholar Pliny described its glowing purple around AD 50.

In the Middle Ages it adorned crowns and bishops' rings, even the sword of Charlemagne, and the Church Father Isidore of Seville interpreted its changing shades as a symbol of the Trinity. Until the 18th century amethyst was a precious stone reserved for wealthy buyers, comparable to ruby or sapphire. That changed with the great Brazilian finds: first in 1727 in Minas Gerais, then in the 19th century above all in Rio Grande do Sul. The huge new finds made amethyst one of the most affordable violet gemstones of all.

Profile

An amethyst druse or a rough crystal at home? I saw, grind and polish it in my workshop in Auggen, as a slice, a flat polished piece or a finished piece of jewelry. The stone processing calculator shows you the price straight away, and the inquiry is non-binding.

Processing notes

With a Mohs hardness of 7, amethyst is slightly harder than agate, jasper or petrified wood and is likewise charged at the rate for hard silicates.

Handling heat is especially important: if too much heat builds up during dry grinding or polishing, the stone loses its violet color at that spot and turns milky to yellowish, the same effect that also produces citrine. That is why I always work amethyst wet and with little pressure. Druse pieces bring a second challenge: the individual crystal tips are only attached at their base and break off easily if handled carelessly, so I saw and grind them with appropriate care.

What I can do with your find or rough piece:

• Sawing: cutting druses open, slices and flat polish blanks
• Grinding and polishing: from a rough polish to a high gloss
• Drilling: for pendants, in solid areas of the stone
• Turning it into jewelry: from found stone to finished piece

You can work out the cost yourself with the stone processing calculator right away. Not sure whether your find is amethyst, colored fluorite or dyed glass? Send me a photo, and I will take a look.

From the workshop

Both images show amethyst from Sulzburg, barely half an hour's drive from my workshop in Auggen. The first piece is a flat polish and shows the mineral association typical of the veins there: radiating baryte at the top, beneath it a salmon pink agate band, then the amethyst crystals, whose tips appear as a zigzag line in the cut. Along the lower edge follow white and reddish growth bands, colored by iron oxide. Stage 4 of the graphic above shows exactly this structure.

The macro shot shows the crystals up close: milky white at the bottom, increasingly violet towards the top, with fine red films of iron oxide in between. At one spot a fine crack inside breaks the light into rainbow colors.

Frequently asked questions

Is every amethyst equally dark?

No. The color comes from iron in the crystal lattice, which is changed from the outside by natural radioactivity. That is why usually only the outer crystal tips, the last to grow, are strongly violet, while the base at the edge of the cavity stays pale. You can see this zoning in almost every piece that has been sawn open.

What is the difference between amethyst and citrine?

Mineralogically both are the same quartz. If amethyst is heated to about 350 degrees, it loses its violet color and turns yellow, becoming citrine. By far the largest part of the citrine on the market is made this way, natural yellow quartz is much rarer.

Does the color of amethyst fade over time?

Not with normal use as jewelry. It is only sensitive to strong heat and to very long, intense exposure to light, such as years directly on a windowsill in full sun. In everyday life this is not an issue.

Where does the name amethyst come from?

From the Greek amethystos, not drunken. The Greeks and Romans believed the stone protected against the effects of wine, which is why it was often made into drinking vessels and rings.

Is there amethyst nearby, in the Black Forest?

Yes, near Sulzburg in the Münstertal valley, a former mining district with hydrothermal baryte and quartz veins. There, pale amethyst sits together with agate and fluorite in druse cavities. The old outcrops, however, are now heavily overgrown and only accessible in places.

Sources

• Amethyst. In: steine-und-minerale.de, mineral encyclopedia.
• Amethyst. In: specialist blog of the Deutsche Gemmologische Gesellschaft e.V. (DGemG).
• Geheimnis der Welterbe-Amethyste gelüftet. In: scinexx.de, Geowissen.
• Sulzburg, Quellen, Geologie und Mineralisation; Sulzburg, Grubenfelder und Gänge. In: clara-mineralien.de, Schwarzwald Süd.
• Achate aus Sulzburg. In: mineralienfreunde-hochrhein.de, reports.