So, there’s this woman we know. She refuses to wear gold. Just won’t do it. One time, she got this ring, and after wearing it for a couple of weeks, she took it off to discover that the patch of skin beneath had turned this weird shade of green. She showed it to us. It was pretty nasty.

It took some convincing, but we managed to get the point across to her: it wasn’t the gold’s fault. See, the ring looked pretty enough, and it was plated in 24 karat (pure) gold, but it was cheap (she paid a lot for it, but it was cheap). It wasn’t a gold ring—it was gold plated.

Hit up your local middle school, and ask to borrow a science textbook. Turn to the page with the periodic table of elements, and look in the middle. You’ll find gold, with an atomic number of 79. To the left, you’ll find platinum (78). Head up a row, and you’ll find palladium and silver (46 and 47). Head up one more row, and you’ll find, nickel, copper and zinc (28-30).

Okay. Enough with the chemistry lesson. Point is, it’s these basic elements, all safely tucked into their own little section of the periodic table, that constitute most jewelry. The higher the atomic number, the rarer the metal, and the more valuable it is.

When you wear a cheap costume piece, like our friend did, it’s easy to get fooled. Most people hear the term “24-karat gold plated,” and think they’re doing okay. After all, 24-karat gold is 100% pure gold. What’s better than that? And who cares if it’s plated? It’ll be shiny, right?

Thing is, the metal underneath the gold is going to oxidize, because that’s what inexpensive metals do when they interact with the acids in our skin, in lotions, or in that onion we’re chopping up. Copper, for example, is a commonly used base for plating, and even the gold on top can’t stop chemistry: over time, the metal will oxidize, leech through the plating, and leave that telltale green band. Fun fact: the Statue of Liberty, when it was assembled, was the color of a penny. It was made of copper, which, over time, oxidized. Same deal.

Anyway. That’s why real jewelry is made of a mixture of metals called alloys, and when it comes to gold (for example), the most common ratio you’re going to find in a high-quality piece is 75% gold, 12.5% copper, and 12.5% silver: otherwise known as 18-karat gold. There are different recipes, but this is basic, 101 stuff.

But wait: Why would you want to pay good money for something that’s not pure? And why are you putting copper in my jewelry? Didn’t we just say that was bad? Good jewelry should be made of 24-karat gold, because it’s the purest, right?

Not quite. The thing about gold is that it’s really soft. You can make a ring out of 24K gold, but the moment you bump it into something, it’ll squash flat, bend, or break, leaving a ruined ring.

As it turns out, making jewelry with 75% gold (aka 18K) is the sweet spot. It’s strong enough to endure wear and tear, but pure enough to remain beautiful and lustrous, making it the go-to for the best jewelry makers. Copper is a cheaper metal, sure, but it’s also extremely durable, and if proportioned properly, won’t cause tarnishing or discoloration. Same with silver.

Now, here’s where it gets fun: it’s that other 25% that makes all the difference. We said already that standard, 18K yellow gold is 75% gold, 12.5% copper, and 12.5% silver (sometimes, a dash of zinc is added for flavoring).

But if we tweak the numbers a little, we get a different alloy. 18K rose gold, renowned for its romantic peachy-pink hue, simply contains more copper: 22%, for instance. 18K white gold can be created using a few different recipes, but usually contains a fair amount of nickel and zinc, or (for more pricy pieces) platinum, in addition to the 75% pure gold. The extra metal essentially “bleaches” the gold, making it white. In other words, if someone offers you a 24-karat white gold anything, just walk away—there’s no such thing, and they’re lying to you.

The same goes for silver. There are a lot of different silver alloys (pure silver is pretty soft as well), but the most common one you’re going to come across is 92.5% silver, with the other 7.5% being—you guessed it—copper, nickel, or zinc. This is called sterling silver, and it’ll be denoted by “925,” somewhere on the piece.