Maybe this is embarrassing to admit, but I have to anyway: for ages, I thought different colors of gold just came from different sources ““ like some gold ore was yellow, some was white; it was the magic of nature. It somehow didn’t occur to me that gold (an element!) wouldn’t be just like a gemstone (not an element! Actually a combination of elements!) and therefore, it could only be one color. It wasn’t until recently that I learned that all the pretty gold colors are the result of thousands of years of humans messing around with alloys.
Alloys are combinations of two or more metals, with the intention being to make the resulting alloy stronger or more resistance to damage (such as corrosion) than the original metal. Gold is a super popular metal, in part because it and copper are the only two naturally occurring metals that are not silver/gray, and in part because oh, how pretty and shiny! It’s also remarkably soft. The biting-the-coin thing people do in old timey movies with pirates and cowboys and dubious old timey business men were not just making this stuff up ““ solid gold should show a bite.
It’s due to this softness that gold has to be alloyed with other metals for use in jewelry: without the alloys, the jewelry would not be durable at all. Pure gold is 24 karats (K), but most jewelry grade gold falls anywhere between 10K to 22K. And here is where fancy jewelry meets math (so hey people who scoffed and said they’d never have to understand proportions and ratios, you do now): the karat value of the gold tells you how much gold and how much other metal you can expect in your jewelry. For example, 18K gold will be 18 parts gold and 6 parts other metal(s), making 18K gold 75% gold, while 10k will be 10 parts gold and 14 parts other metals, making it only 41.7% gold. Anything below 10K cannot be sold as gold in the United States, and with good reason ““ the gold content in those metals would be very low indeed.
As anyone who has ever looked at the jewelry in a department store can attest, the current most popular colors of gold used in jewelry are yellow gold, rose gold, and white gold. The color of these different types of gold comes from the alloy. Yellow gold is alloyed with copper, silver, and zinc, rose gold is predominantly alloyed with gold and copper (the rose color coming from the pretty red of the copper), and white gold is generally made with either gold and palladium, or gold and nickel. While various metals may work to create the desired color, very specific combinations and proportions must be chosen to ensure that the metal is not too brittle to be workable.
But while those colors are the most popular, they’re not the only gold color options out there: to my surprise and pleasure, it turns out there is green gold, blue gold, and purple gold, too. Green gold is an alloy of gold and silver, blue gold comes from gold and iron, and purple gold is made when gold and aluminum find themselves thrown together. I’ve seen green gold used in accent jewelry before, but never blue and purple. It turns out that there is a reason for this: blue and purple types of gold are extremely brittle. They can be used in small accents, or faceted and made shiny, but they’re not ductile or malleable enough to be used large scale. However, the fact that so many different color options exist, and that they can be made through relatively simple metal combinations (just one or two metals, not, you know, 15 all thrown together) is just wildly cool to me.
So the next time you put on a pair of earrings or a ring, think about the science and chemistry that made your jewelry look so good. I know I will.