Complex sentence: Pewter with a high lead content became common by the dark ages, and was considered highly valuable by the 12th century, AD; only royalty, merchants, clergy, and the like tended to have or use it.
Split into simpler sentences: Pewter with a high lead content became common by the dark ages, and was considered highly valuable by the 12th century, AD. At that time, only royalty, merchants, clergy, and the like tended to have or use it.