Slovak opal mines were the first and for several centuries the only source of expensive opal in the world. Because of its scarcity, opal was considered the “King of Gems” and the rarest gemstone in the world. The first written mentions of opal mining come from 1597, but according to the mining methods used, deep mining probably started in the 11th or 12th century. Surface mining, however, dates back to Roman times as early as 200 BC.