Complex sentence: Iberian tin was widely traded across the Mediterranean during the Bronze Age, and extensively exploited during Roman times, but its tin deposits were largely forgotten throughout the medieval period and only rediscovered in the 18th century and only re-gained importance during the mid 19th century.
Split into simpler sentences: Iberian tin was widely traded across the Mediterranean during the Bronze Age, and extensively exploited during Roman times. But Iberian tin deposits were largely forgotten throughout the medieval period, were not rediscovered till the 18th century, and only re-gained importance during the mid- 19th century.