a German town ran out of agate, so it sent its sons to Brazil, and they shipped rocks home as ballast

Idar-Oberstein is a town in the Hunsrück that has been cutting stone since at least the fifteenth century, on the back of local agate and jasper deposits in the surrounding volcanic rock. Water-powered grinding mills along the Nahe, cutters lying face-down on wooden benches to hold the stone against enormous vertical sandstone wheels — a posture that wrecked a lot of spines over a lot of generations.

By the nineteenth century the local deposits were essentially finished.

What happened next is the part I find genuinely moving. Emigrants from the region who had gone to southern Brazil found enormous agate deposits in Rio Grande do Sul. The material started coming back — reportedly, in part, as ballast in ships that would otherwise have sailed light. A town's entire industry was saved by its own diaspora sending home the raw material.

Idar-Oberstein is still a cutting centre. It never went back to being a mining town; it became a processing town, which is a different and more durable thing.

The other legacy is dyeing. The Brazilian agate was often pale and unremarkable, so cutters there developed the treatments that are now standard across the whole industry — soaking porous bands in sugar solution and then in sulphuric acid to carbonise it black, iron salts for reds and browns, and so on. Banded agate takes dye unevenly, because the bands differ in porosity, which is precisely what makes the result look natural.

Almost every black onyx you have ever seen is dyed grey agate. It has been done this way for two centuries, it's disclosed in the trade as a matter of routine, and essentially nobody at the retail end knows.

I have a dyed blue agate slice that cost four dollars and a natural grey one from the same kind of deposit that cost about the same and looks like a puddle.

I like the puddle better now, which I recognise is a position I've grown into rather than one I started with, and I try not to be a snob about it in shops.