I've found a tile which seems to contain Porcelain:
I'm not quite sure how it happened; it seems like a city-state used to be on that tile, but it was somehow removed (I thought cities which are city-states can never be razed). In any case, Is there any way I can add this Porcelain to my luxury resources?
Although it's in my territory I don't seem to have it currently, and the tile doesn't list any improvement it needs to connect it.