Before the creation of the Empire, the Romans were also working copper at Madjenpek in Jugoslavia, as well as mines in Asia Minor. They were in fact inveterate seekers after metals, but especially gold, silver, copper, tin and lead and at a later date also iron. Apart from the immense deposits of cupriferous yellow ore in Spain, the principal copper ore was malachite, after the much less abundant native copper had been exhausted. The Romans may also have used some of the very noticeable greenish-blue chrysocolla (copper silicate), and perhaps a little of the very rich ore chalcocite, which is often grey or black, and sometimes soft and sooty (copper glance). The great basis of Roman supplies was Spain. The sulphide of copper found in the Spanish mines is bright yellow chalcopyrite; it contains almost equal parts of copper, sulphur and iron. It must be smelted, whereas the others can readily, although slowly, be leached out of their stony matrix. Thus, quite a long time ago the old metallurgists must have encountered some tough problems; but in this case the remarkable purity of the Spanish slags, of which enormous quantities still remain, testifies to their success.