Turquoise has been known by many names, but the word turquoise, which dates to the 16th century, is derived from an Old French word for "Turkish", because the mineral was first brought to Europe from Turkey, from the mines in historical Khorasan Province of Iran. Pliny the Elder referred to the mineral as callais, the Iranians named it "phirouzeh" and the Aztecs knew it as Teoxihuitl. Turquoise was among the first gems to be mined, and while many historic sites have been depleted, some are still worked to this day. These are all small-scale, often seasonal operations, owing to the limited scope and remoteness of the deposits. Most are worked by hand with little or no mechanisation. However, turquoise is often recovered as a by product of large-scale copper mining operations. For at least 2,000 years, Iran, known before as Persia, has remained an important source of turquoise which was named by Iranians initially "pirouzeh" meaning "victory" and later after Arab invasion "firouzeh". In Iranian architecture, the blue turquoise was used to cover the domes of the Iranian palaces because its intense blue colour was also a symbol of heaven on earth. Hardness and richness of colour are two of the major factors in determining the value of turquoise; while colour is a matter of individual taste, generally speaking, the most desirable is a strong sky to "robin's egg" blue (in reference to the eggs of the American Robin). Whatever the colour, turquoise should not be excessively soft or chalky; even if treated, such lesser material (to which most turquoise belongs) is liable to fade or discolour over time and will not hold up to normal use in jewellery.