The catalog from which I took the tiles mentions their manufacturing process in passing:

> The colours are at least one-quarter-inch thick, backed up to a full thickness of seven-eighth inch thick of pure granite sand and Portland cement, subjected to a high pressure in steel moulds, after which the tiles are immersed in water for a certain period, to render the material thoroughly sound, in the usual way adopted for a concrete of the highest quality. Being so constructed, the material — after being thoroughly matured — is quite impervious to moisture, is frostproof, and is eminently adapted for any class of flooring, — particularly where great durability is of utmost importance.

I wonder if I'll ever come across any in real life in some old building in Europe and if their hope of "great durability" extended to 120+ years :-)