there's one mine on earth producing a crystal that is purple at one end and yellow at the other

Ametrine. A single quartz crystal, part amethyst, part citrine, with the two colours meeting in a zone that can be startlingly sharp — one face purple, the adjacent face gold, with a diagonal boundary you could draw with a ruler.

Essentially all of it comes from one place: the Anahí mine, in the Santa Cruz department of eastern Bolivia, in swampy remote country near the Brazilian border.

The accepted explanation involves a temperature gradient across the growing crystal. Amethyst and citrine differ by the oxidation state of iron in the lattice and the colour centres associated with it, and where part of a crystal grew or sat slightly hotter than the rest, that part ended up yellow while the cooler part stayed purple. The boundary is a growth-sector boundary, which is why it can be geometric rather than blended.

There's a foundation legend attached, involving a Spanish conquistador in the 1600s, a marriage to a princess named Anahí, the mine given as a dowry, and her death — the stone's two colours read as her divided loyalties. It's told everywhere, and I have not been able to find anything supporting it earlier than the modern marketing of the material, so I'd treat it as a good story rather than a history.

Synthetic ametrine exists and is common, and heat-treating amethyst partially can fake it, so the trade takes provenance seriously.

What I actually want to say about it.

I have felt two contradictory things about the same person simultaneously and continuously for eleven years, and I used to treat that as a problem to be resolved. I assumed one of the two was the real feeling and the other was noise, and that with enough honesty I'd work out which.

I have not worked out which, and I no longer expect to, and things have improved considerably since I stopped trying.

Mine is a small faceted rectangle, thirty-four dollars, and the boundary runs corner to corner.

It is not half a good crystal and half a compromised one. It's one crystal that grew in a place where the temperature wasn't the same all the way across, and it came out as an accurate record of that, and it is worth more than either colour would be alone.