The first coins to appear in the Western world were issued by the Indians and the Ionian Greeks in the eighth century B. C. These coins, which were made of electrum, a natural combination of gold and silver, were irregular in weight and quality. The pure gold and silver coins with related values which appeared during the reign of Croesus (560 -546 B. C. ) provide the first un doubted evidence of standard coinage by state authority. The coins were not perfectly shaped, however, for they were struck with a hand wielded hammer. The trend toward complete mathematical symmetry did not, in fact, begin until the coining press, invented by Leonardo da Vinci in the sixteenth century, was generally used in the middle of the seventeenth century. One should not assume, however, that only machine made coins are prized for their workmanship. The silver dekadrachm from Syracuse, struck about 413 B. C. , is considered one of the finest Greek coins and is worth more than a thousand dollars today. An artistic masterpiece of a much later period is the silver taler minted in Ratisbon, southern Germany, in 1754. The value of a coin is not primarily determined by its age, as many people seem to think. Many Greek and Roman coins that were issued in abundance can be purchased for a moderate price. On the other hand, a German coin made of shrapnel during the First World War is very rare and valuable. Among the especially rare United States coins are the 1804 silver dollar, the 1822 five-dollar gold piece, and the 1894 silver dime. The reason some old Roman coins are NOT costly is that they are ______.