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Amethyst Cathedral Geodes: From Lava Tube to Living Room

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: an amethyst cathedral is a tall, narrow section of a geode, usually from the basalt mines of Rio Grande do Sul in Brazil, cut open along its length so the purple crystal chamber faces the viewer. Before sale it is sawn flat at the base, set in weighted concrete so it stands upright, and its outer rind is cemented and painted. Judge one by crystal color and size, an undamaged cavity, honest repairs and a stable base.

What makes a geode a "cathedral"?

The name describes shape, not a mineral or a locality. A cathedral stands taller than it is wide, with an arched opening that frames a hollow lined in amethyst points, which is where the comparison to a church nave comes from. Most are sold as one half of a split cavity, and matched pairs, the two halves of the same geode, are sold together for a symmetrical display.

That shape comes from the lava. Geology.com explains that many cathedral geodes are branches of small lava tubes, cavities that can be close to a meter across and many meters long, later filled with agate and quartz. Research at Ametista do Sul by Proust and Fontaine found the geodes there vertically elongated, and traced the metre-sized branching forms to tube-like cavities that merged as they grew. Round bubbles give round geodes; elongated, branching ones give cathedrals.

Inside, the layers run in a fixed order. At the Triz quarry in Ametista do Sul the wall goes, from the outside inward, celadonite, chalcedony, fine-grained quartz and then large amethyst crystals, as recorded by the same authors in a second study. On a cut cathedral those layers show as a thin green skin, a gray-blue agate band and then the crystals.

How is a cathedral made, from tunnel to showroom?

A cathedral is natural on the inside and manufactured on the outside. A 2011 tour report in Europa Star followed geodes from the mines of Rio Grande do Sul through a processing warehouse. The steps:

  1. Locating and extracting. In the tunnels, a miner who finds a geode drills a small hole to judge its shape and quality, then chisels it out of the basalt, which can take hours or even days.
  2. Reinforcing. Because geodes break easily, they are reinforced with rebar at the warehouse, and the steel is cemented in place.
  3. Sawing and polishing. With the steel holding everything together, a large diamond-tipped saw cuts the geode, and the cut faces are polished to a mirror finish.
  4. Coating the back. Cement is spread over the rough exterior to hide the rebar, then stippled with a moist brush to give it texture.
  5. Repairing. Pieces broken during extraction or in transport go to the "clinic," where workers fill holes with small pieces of amethyst glued into place like a three-dimensional puzzle.
  6. Painting and washing. The cemented areas are painted matte black, and the geode is washed with a high-pressure hose before sale.

Geology.com describes the same finishing for display cathedrals: a sawn flat base filled with metal-weighted concrete so the piece stands upright, sanded edges around the cavity, and paint over the whole exterior to hide scrapes from collection and transport. The black or dark gray back on nearly every cathedral is therefore cement and paint, not basalt. That is standard practice and not a fraud, but a seller should not describe the shell as natural.

Which minerals are sold as cathedrals?

  • Amethyst is the classic. Brazilian basalt supplies most of the world's tall cathedrals, while Uruguayan material, compared in the Uruguay vs Brazil guide, is more often sold as clusters and smaller cut pieces.
  • "Citrine" cathedrals are heated amethyst. Geology.com notes that the large citrine geodes from Brazilian basalt, sold at two, four, six feet tall or more, are almost always found full of purple amethyst and converted in ovens.
  • Agate and quartz cathedrals come from the same flows when the cavity filled with banded agate or clear quartz instead of amethyst.
  • Calcite companions. Golden or white calcite crystals, including pointed "dogtooth" scalenohedra, often grow on top of the amethyst late in the fill. A cathedral with a well-formed calcite group is a collector piece rather than plain decor.

How do you judge a cathedral's quality?

Work from the inside out, then check the outside.

The crystals

  • Color depth and evenness. The deepest purple usually sits at the crystal tips; Mindat notes that in geodes color is often most intense in the growth zones under the rhombohedral faces. Look for color that holds across the whole chamber, not one dark patch.
  • Point size. Larger, well-separated points with sharp terminations rate above fine druzy. Even so, amethyst crystals stay modest in size; Mindat calls crystals longer than 30 cm very rare.
  • Luster. Bright, glassy faces. Dull or frosted tips can mean acid cleaning or wear.
  • Brown or orange tints. Check where the cut runs through the crystals. Brown can indicate iron staining or partial heating.

The cavity

  • Depth and opening. A deep cavity with a clean arch reads better than a shallow scoop.
  • Damage. Chipped tips at the rim are common. Look into the back of the chamber for missing crystal patches.
  • Repairs. Under a flashlight, glued-in pieces often show as a group of crystals at an odd angle, a hairline seam, or a film of glue that does not sparkle. Small repairs are normal on large pieces; they should be disclosed.

The shell and base

  • Stability. It should stand without rocking on a level floor. Rap the base lightly; a hollow sound or cracking concrete is a warning.
  • Coating condition. Flaking paint or crumbling cement can be refreshed, but loose rebar ends cannot be ignored on a heavy piece.

What sizes are available, and what drives the price?

Cathedrals run from desk pieces under 30 cm and a few kilograms to floor pieces taller than a person. GIA notes that Brazilian amethyst geodes can be so large you can stand in them, and the Uruguayan district studied by the University of Göttingen yields giant geodes sometimes over 5 m high. Geology.com puts many large cut geodes at thousands of dollars each.

Height, weight, color grade, crystal size, damage and freight all move the price. The amethyst geode pricing guide works through each driver with numbers.

How should you display and care for a cathedral?

  • Floor first. A large cathedral can weigh tens of kilograms. Place it on the floor or on furniture rated for the load, away from walkways where it can be knocked.
  • Light it from the front and above. A spotlight angled into the chamber brings out depth. Keep hot halogen lamps at a distance.
  • Keep it out of strong sun. GIA notes that some amethyst fades with prolonged exposure to strong light. A sunny window is the most common way a cathedral loses color over the years.
  • Dust, do not scrub. Use a soft brush or compressed air. Quartz is 7 on the Mohs scale, but crystal tips still chip on contact, and water left in the cement shell can stain it.
  • Move it by the base. Never lift by the rim of the opening, which is the weakest part.

Choosing an amethyst cathedral or large geode

Ask for the height, weight and a photo of the back and base, not just the chamber. Ask whether there are repairs, and whether the piece ships by freight. Then compare color and point size against the price.

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Frequently asked questions

Is the black outside of an amethyst cathedral natural?

Usually not. Processors coat the rough back with cement, texture it and paint it black or gray to hide steel reinforcement and transport scuffs. The crystals inside are natural; the shell finish is a standard manufacturing step.

Are amethyst cathedrals glued together?

Many large ones have some repairs. Broken areas are often filled with loose amethyst crystals glued in place, and steel rebar inside the cement holds weak geodes together. Small, disclosed repairs are normal; extensive hidden rebuilding should lower the price sharply.

How heavy is an amethyst cathedral?

It depends on size and wall thickness. Desk pieces weigh a few kilograms, and floor-standing cathedrals can weigh tens of kilograms or more, much of it basalt rind and concrete base. Ask for the exact weight before arranging delivery.

Where do amethyst cathedrals come from?

Mostly from the basalt mines around Ametista do Sul in Rio Grande do Sul, southern Brazil, where geodes are tall and branching. Uruguay's Artigas district produces giant geodes too, but more of its output is sold as clusters and smaller pieces.

Can an amethyst cathedral go outdoors?

It is not a good idea. Sun fades amethyst over time, rain and frost attack the cement and paint, and water trapped in cracks can widen them when it freezes. Keep cathedrals indoors.

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