"I heard" that gray lines forming on porcelain indicate usage of aluminium in the coating. I would primarily like to verify this information.
Bowl #1 and bowl #2 have been used both for several months and were bought in different shops. The same observation goes for other dishes. Some don't feature lines, other dishes feature those after days already. The dishes who tend to display those patterns have been bought all at the same place (a large chain of stores specializing on furniture). The lines are most likely caused by cutlery as in cups the same lines are found forming ellipses at the bottom from the stirring. Again some cups don't feature those patterns others do.
Bowl #1 with no lines despite usage for a while:
Bowl #2 with lines:
Bowl #2 with lines - picture taken HDR mode: