Material & Origin
The rock is 134 million years old. The crystal inside it is not.
Almost every amethyst cathedral in almost every American mall came out of the same lava flow in southern Brazil. This is what happened to it.
- 01
A bubble in cooling lava
About 134 million years ago a flood of basalt buried what is now southern Brazil — well over a million cubic kilometres of it. As the lava cooled, trapped gas left cavities behind. Empty rooms inside solid rock.
- 02
Then, much later, water
The cavity sat empty for a very long time. Eventually groundwater carrying dissolved silica worked up through the flow from the aquifer beneath and began filling those rooms from the walls inward.
- 03
At about the temperature of a hot bath
Not fire, which is what almost everyone assumes. The evidence puts amethyst growth somewhere between roughly 50 and 120 °C, most likely around 80 to 90. Closer to a kettle than a furnace, and unimaginably slower.
- 04
In this order, every time
Green celadonite against the basalt first. Then banded agate. Then colourless quartz. Then, last and innermost, the amethyst. Cut a geode anywhere in the district and the sequence is the same, because the same water did the same thing.
- 05
One flow, twenty kilometres
The source is not a country. It is a single subhorizontal flow forty to fifty metres thick, along a twenty-kilometre strip of hills at Ametista do Sul, Rio Grande do Sul. The mines have names — Salles, Célio, Testa, Trock, Narciso, Mioto, Zanon.
- 06
One eruption, four names
The same lava province is called Serra Geral in Brazil, Arapey in Uruguay, Alto Paraná in Paraguay and Posadas in Argentina. Artigas material from the Uruguayan side runs a deeper violet, with a thicker agate lining and shorter points.
How we label a specimen
- Species
- Quartz — variety amethyst
- Formula
- SiO₂, coloured by trace iron and irradiation
- Locality
- Ametista do Sul, Rio Grande do Sul, Brazil
- Host rock
- Serra Geral basalt, Paraná flood province
- Host age
- ≈134 million years
- Treatment
- None
And what we cannot tell you
We can name the district for most of our amethyst and the mine for some of it. We cannot do that for everything in the building, and we are not going to pretend otherwise. Where the chain runs out, the card says so — which is the only version of this that is worth reading.