tanzanite comes from one hill and current estimates say it’ll be gone within a few decades, so we’re the only people who’ll ever have it

Tanzanite is blue-violet zoisite. It comes from a strip of ground a few kilometres across in the Merelani Hills, near Arusha in northern Tanzania, at the foot of Kilimanjaro. That is the entire world deposit and there is no reason to expect another, because the geology that produced it — a very specific set of metamorphic conditions involving vanadium — is understood to be a genuinely unusual coincidence.

It was found in 1967. The credit is contested; a Maasai herder named Ali Juuyawatu is usually named as the finder, and a prospector called Manuel de Souza as the man who brought it to market, and the details of who told whom what vary depending on who's writing.

Tiffany named it and marketed it, on the reasoning that "blue zoisite" sounded too much like "suicide" to sell.

Almost all of it is heat-treated. The rough is usually brown-ish and reddish; heating to around 500–600°C converts it to the blue-violet everyone knows. It's a stable, permanent, universally disclosed treatment, and untreated blue material exists and is rare and costs accordingly.

The exhaustion estimates get quoted a lot and vary enormously — I've seen anything from twenty to a hundred years, from sources with obvious interests in either direction. Nobody actually knows. What isn't in dispute is that the deposit is finite and small and that mining has been going for less than sixty years.

Which means there's a window. Humans existed for a very long time without this stone and will exist for a long time after it's gone, and the entire period in which an ordinary person can walk into a shop and buy one is maybe a century long, and we're in it.

My aunt has a tanzanite ring she bought herself in 1998 after a divorce, which is the least subtle purchase in family history and which she is completely unrepentant about.

It's a good stone. Slightly included, properly violet from one angle and blue from another because tanzanite is strongly pleochroic — different colours down different crystal axes.

She's seventy-one and has told me, more than once, that it's mine.

I've told her I'd rather she kept wearing it, which is true, and also I've looked up the auction prices, which is also true.