Why do coins change color?

When coins change color it is called toning. Toning is caused by a chemical reaction between the metal (or metals) in the coin and any elements the coin comes into contact with.

Those elements can be found in the air, in the oils on your hands, in the moisture on a humid day, or in other coins.

When a coin naturally tones it can increase the coin’s numismatic value, however, artificial toning decreases a coin’s numismatic value.

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Gold Coins

The more gold a coin has in it the less likely it is to tone because gold is non-reactive. That means gold coins will not oxidize (rust) and are very unlikely to tarnish unless they are combined with a more reactive metal.

Sometimes old and exposed gold coins take on a dark cast that we call gold tarnish, but it is not a chemical reaction and is easy to remove without damaging the value of the coin. Gold toning is very rare and adds considerable value to high karat gold coins.

Gold coins with as little as 10% copper might tone. The toning would show as a green cast in the coin, and in actuality, it is the copper that is toning rather than the gold.

Rose gold coins like the brand new 2022 UK Sovereign will tarnish and tone over time. Rose gold is a mix of gold and copper. It is the copper that makes the coin look pink. Copper is a reactive metal and even the Sovereign at 916.67 percent gold, looks undeniably rose-colored and will tone over time.

Silver Coins

Silver is a reactive metal so it tarnishes and tones in a variety of ways. Most natural silver toning is a result of exposure to oxygen or to sulfur.

Toning occurs in stages, rather than all at once. Slight toning on a silver coin caused by oxygen will appear as a pale blue or gray cast over the toned area of the coin. As time passes the toning develops a richer hue, getting bluer, or grayer. Sulfur caused toning of a silver coin will result in blues, pinks, and sometimes yellow and green colors.

When a toned coin includes distinct pink, blue, and green toning it is considered a rainbow-toned coin. Rainbow toning is the most desirable type of toning.

Copper coins

So far we have discussed oxygen and sulfur toning, but most copper toning is caused by the oil in your hands.

Copper is a reactive metal that oxidizes and tones easily. When a copper coin is toned it often appears darker and shinier than it did in its original condition. Sometimes copper coins can incur a colorful cast, so that the coin looks like it is a different color when held at an angle.

Copper is usually reserved for low face-value coins and minted in such quality and quantity that the coins are often damaged in circulation. Made of a reactive element, the copper coins are more susceptible to oxidization. Green patina is copper rust. Oxidization can cause pitting and it can mar the surface of the coin, greatly diminishing its numismatic value or destroying the coin completely.

Copper-Nickle Coins

Today, the great majority of silver-color circulating coins are made of cupronickel. These coins can tone too. When an older cupronickel coin toned it was often the result of a harsher chemical found in the coin holder. Coin envelopes were not always made of acid-free paper and many plastic coin holders (and blister packs from the mint) contained sulfur.

Silver-clad cupronickel coins can tone in a vibrant fashion because it is the silver that tones.

Cupronickel toning begins with a golden cast. It changes slowly, becoming more vibrant and less silver-looking, becoming more beige-hued over time.

Gold-color Coins

Brass is made of nickel and zinc, and gold-color coins are often made of nickel and brass, or aluminum and brass. None of these elements are highly reactive so it takes a long time to see a little toning on a brass coin.

Brass coins can obtain darker golden tones, or more yellow tones depending on their environment. Very old brass coins can appear copper-color, or different shades of brown. Typically the value of brass coins is in their details rather than in their toning.

Water exposure can cause brass coin oxidation. The oxidation will appear pink or red and if it has damaged the surface of the coin it can greatly reduce the numismatic value.

Toning is a boon to some collectors and a bane to others. There are different types of toning, and different grades of toning, and different ways to appraise both. We’ll have to cover those details in another post!

As with so many things, beauty is in the eye of the beholder. If you like the look of a specific coin then get it, watch it change over time, and enjoy your investment.