Silver has long been valued as a precious metal for coinage. The metal is found in the earth's crust in pure, free elemental form, as an alloy with gold and other metals, and in minerals. Most silver is produced as a by-product of the refining of copper, gold, lead and zinc. Silver exhibits the highest electrical conductivity, thermal conductivity and reflectivity of all metals. Silver metal is used in many coins, in bullion, sometimes together with gold: although it is more common than gold, it is much less common as a bullion. Silver coins are considered the oldest mass-produced form of coinage. It is believed that as a means of exchange, coins first appeared in Ancient China, and later in Asia Minor and Europe they began to be made of precious metal as a guarantee of their value. Silver has been used as a metal for coins since the time of the ancient Thracians, their silver coins were a popular medium of trade that was later used by emerging nations in Europe. The ancient Persians in Asia also used silver coins. The monetary system was adopted by the Persian Achaemenid dynasty via the Thracians in Phrygia (Blegia). In the past, British pennies were also made of silver. Silver coins are also used today as a means of investment against inflation. As with all collectible coins, many factors determine the value of a silver coin, such as its rarity, demand, condition, and the number originally minted.