Its a tiny bit of, I assume, steel. I couldn’t find it. The mechanics at the local bike shop couldn’t find it after turning the tyre inside out. I was in town at an appointment, and coming out and finding my tyre was flat again, I had a little, internal, meltdown.
The mechanic at the shop I bought the bike from, however, spent some time, and found it. A tiny wire.
So, lesson learned: get help when you need it. The advice I got at the shop suggested that these are really common around Christchurch, and that most bike tyres have a few in them. I think that a new tyre, a more robust one than the one that came with the bike is probably called for.