No gold is 100% pure, unlike the floor of the New Jerusalem. All gold contains other metals, such as copper and silver, as impurities. Gold is determined as "pure" when it is pure enough to used for manufacturing new products, such as jewelry or electronics. Today, "pure gold" must be from 99.95% (three nines five) to 99.99% (four nines) pure. Gold must be "pure" in order to be sold at near market price. In rare cases, 99.999% (five nines) or 99.9999% (six nines) gold is needed and this is sold at a premium.
Generally, gold is determined as "pure" by it's appearance in a bar form. Freshly poured and cooled "pure gold" bars will have a brilliant deep gold color, with NO evidence of discoloration (usually red). On the top surface, you will see a "sink", a place where gases are expelled and the gold has sunk a slight bit. Radiating out from the sink, there are many squarish "crystals", about 1/8" to 1/4" in size. Impure gold has none of these characteristics. Nothing else looks like pure gold.
Sometimes, people buff the bars, in order to make them look purer than they really are. In such cases, the buyer should remelt and repour the bar before he buys it.
When the gold is molten in the crucible, one can tell when it is pure by looking at the surface. It should look very clean and brilliant. There should be no film on the gold. Slightly off-purity gold can many times be made pure by adding periodic pinches of sodium nitrate (niter), sodium carbonate (soda ash), and sodium borate (borax). The niter oxidizes the impurities and the other chemicals collect them and bring them to the side, where they are removed with a carbon rod. Eventually, there is clean, brilliant gold, with no film. Sometimes, very pure gold seems to cast a greenish glow.
http://www.finishing.com/249/36.shtml
Sorry, I don't have a picture!
