Physics

Generally whenever I am packing objects for moving or storage, I ensure that the smooth side of one item faces the smooth side of the other. E.g. if I have a box of old smartphones or tablet PCs, putting the faces together to have screen against screen (and no debris of course) works well. No need for individual wrapping.

Ceramic tiles from the shop are always boxed facing the same direction. It seems every single factory is doing this without exception. The rough ceramic bottom is stacked against the smooth glazed side.

So when storing excess spare tiles for future repairs, I restack them smooth side against smooth side. But for one batch of tiles the stack was a bit rocky - less stable. Whereas the original stack of all facing the same direction does not rock. Apparently they are less flat or less symetric than they appear.

Is glass harder than ceramic? Or did they decide that compromise is better than a risk of fractures due to the lack of flatness?

IIUC, the bottom of a tile is 6.5—7 Mohs and glass is softer. But different kinds of glass are different and I don’t know where the glaze of a ceramic tile stands on that scale. IIRC, glaze on ceramics is a kind of glass, or at least glass-like.