Combinations of gold and other metals. Gold is not oxidized in air, and for thousands of years was the only metal available that remained bright indefinitely. This, coupled with its pleasing yellow color, made it cherished for personal adornment and other decorative uses and valuable as a medium of exchange. The high specific gravity of gold (19.3) also gave some insurance against dilution with cheaper metals of lower density. Pure gold is soft. The addition of copper hardens the gold, and ultimately gold-copper alloys became standard for coinage. Gold coins in the United States contained 10% copper, the balance gold. Prior to 1934 the value of coinage had been based on the price of gold. In that year coinage of gold ceased in the United States, and gold was priced at a market value. The United States Mint sold gold to licensed users for industrial and ornamental uses from 1934 to March 17, 1968. From this date to January 1, 1975, some banks and one broker were licensed to sell gold, but only to licensed users for industrial and artistic purposes. Since 1975, there have been no restrictions on the buying and selling of gold.