Gold

Gold
The beauty and rarity of gold, and the fact that it does not rust or tarnish, have made it highly valued throughout history. Pure gold is soft. It is generally hardened by alloying (mixing) it with copper
or silver. The gold content of these alloys is measured in carats. Pure gold is said to be 24 carats, whereas an alloy of equal parts of gold and, say, silver is said to be 12-carat gold. Eighteen-carat gold
is 18/24 pure gold; that is, 75 per cent gold. Mining and panning for gold Gold is found as a pure metal. It occurs either in the form of grains in sand or gravel, or as veins in rock. Very occasionally, much larger pieces are found on the surface. Piece Nowadays, most gold is obtained from mines. The rock is blasted out with explosives and then crushed by heavy machinery. The powdered rock is then treated with chemicals to extract the gold.
Grains of gold are found in the beds of some rivers or streams. This gold can be collected with a shallow metal pan in which the sand and water are carefully swirled around. The sand is washed over the edge while the gold, which is much heavier, sinks to the bottom. There is plenty of gold in the sea, but vast amounts of sea water would have to be treated to obtain even a little gold.  So far, no one has found a way of doing this cheaply.
(Oxford Children Encyclopedia 1992)