Pure Gold is Dirt!
This is an image of gold “sponge” after precipitation from aqua regia, which is a mixture of hydrochloric and nitric acids. Because of the moisture you’ll notice the brownish hue.
When we think of gold, most of us think of shiny yellow colored jewelry. What most people don’t know is that pretty shiny jewelry is not how gold always looks. During the refining process it actually takes on the appearance of dirt with a brown hue. When gold is not metalized, its natural yellow color appears brown even though it’s as yellow as gold can be. The primary reason is that it's holding some moisture.
Once gold has been refined to a fineness of 99.99% (referred to commercially as pure or fine), it can be used to make any alloy a fabricator chooses. For instance, a lot of jewelers take fine gold and add a pre-mixed alloy (mixture) of copper, zinc, silver and possibly nickel, to make the material stronger for this application. The amount of actual pure gold in an alloy is measured by karat (K). The more gold in the alloy the higher the karat. The most typical are listed here with the % of pure gold in each respective karat measurement.
10K – 41.6% gold
14K – 58.3% gold
18K – 75% gold
22K – 91.6% gold
24K – Pure gold
After the annealing process - removing any moisture - gold starts to take on it’s normal golden hue. Once it is melted it takes on it’s golden luster.
Class rings are almost always 10 karat because they are not as pure, thus, much less expensive. A gold Rolex is made with18 karat gold and is much more expensive. The less gold used, the less expensive the piece of jewelry is. No one wants to spend the price of a gold Rolex on a high schooler's class ring.
The reason jewelers use this alloy is to add hardness and color. Pure gold – 24 karat – is very soft. While very rich and bold, it would dent or nick very easily with normal daily wear. Although, in India it is tradition for jewelry to be 24 karat.
Fun fact –
Precious metals are weighed in "troy ounces" instead of "avoirdupois", which is the measurement system for standard pounds and ounces.
You may have noticed while looking at your jewelry over the years, there is a marking on the inside. Jewelers will use either 750 or 18K to signify that it’s 18 karat. Now that you know what percent the gold content is, you can easily figure out the actual gold value based on weight.
For example – If an 18 karat ring weighs .25 of a troy ounce, and 75% of that .25 of a troy ounce is gold, then there is .1875 of a troy ounce of gold in that ring (multiply .25 by .75). If gold was trading at $1,200 a troy ounce then you can figure out that there is $225 worth of gold in that ring (.1875 x $1,200).