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I've never heard that about cobblestone, but Roman concrete and "Damascus" steel are other examples. Granted, we've recently (mostly) rediscovered how to make

• philwelch 6y ago
  There’s an important distinction here between losing the knowledge of how X people made Y, and losing the knowledge of how to do Y. For instance, the secret of Greek fire was lost to history, but I’m pretty sure napalm does the job just fine these days. As for Roman concrete, the Romans were blessed with some really nice volcanic ash, but we can build things out of concrete that the Romans never could thanks to rebar alone.