Natural (Placer or Nugget) Gold Purity, Explained

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Written By Monica

How pure is your natural (placer) gold? Maybe less than you think…

You found an ounce of placer gold! Congratulations!

Now here’s a math question: How much is that troy ounce worth at today’s gold price of $1,700?

When I sold the first gold I made as a professional dredger I got hit with the bad news that every prospector and miner eventually gets: natural gold is not pure gold.

In fact, at $1,700 per troy ounce that gold could be worth as high as $1,615 or could be as low as $850. It all depends on the purity of the gold.

I wrote this guide to tell you exactly what you need to know about gold purity so you know the worth of your find!

Natural gold is always hanging with friends

When asked what gold purity is, Dr. Erik Melchiorre, Professor of Geology and an expert on placer mining at California State University – San Bernardino, explained, “First, let’s define the difference between gold and ‘gold’. Gold is the pure metal but since no nuggets are pure, ‘gold’ found in nature is actually an alloy of gold, silver, and/or copper with gold being the majority metal.”

That’s why gold looks so different from different areas! The color is affected by the amount of copper and silver that’s mixed in.

Here’s a simple color spectrum chart that explains why your gold looks the way it does.

Gold Purity in Alaska

When gold is mixed with too much silver or too much copper, it stops being gold and becomes something else. It can even sometimes form an alloy naturally with its friend silver.

Electrum: The Natural Gold-Silver Alloy

Electrum is the official name for any naturally-occuring alloy of gold and silver that’s 20-80% gold and 20-80% silver.

For some reason beyond my understanding only silver can alloy, or intermix, with gold naturally in sufficient quantities where you can find specimens that are half silver and half gold. Copper, while almost always found in naturally occuring gold, is only present in small amounts.

Even at 20% silver the alloy becomes so unrecognizable from gold, that often prospectors don’t know what they’re looking at and toss it away.

Electrum Gold and Silver Alloy

But electrum is incredibly rare, and is mostly found in gold mining regions of Turkey or the state of Nevada in the U.S.

Rose Gold: Manmade Gold-Copper Alloy

Rose gold is the most common name for the alloy of gold and copper to achieve a golden look with a reddish hue. Rose gold is entirely manmade and does not occur naturally! So we won’t talk about it too much here.

Half-Breed: The Bound Silver-Copper Nugget

In the Upper Peninsula of Michigan, around old copper mines, you can find natural nuggets that are a mix of copper and silver.

For some reason locals call these “half-breed nuggets.”

It seems that the silver and the copper adheres together strongly, but doesn’t alloy the same way gold and silver does.

100% pure gold actually doesn’t exist

The purest gold in the world is 99.99999% pure. That’s what you find stamped on gold bullion.

pure gold bars

That level of purity is achieved through the refining process, and it’s the best we can do with the laws of physics.

Currently, there isn’t a viable method to produce 100% pure gold due to the fact that it would cost more than the gold’s value to remove those last impurities on a molecular level. 

Defining purity: karats and fineness, explained

The best known methods for defining the purity of gold is by Karat and Fineness. 

Often when people hear the term “karat” they think of gemstones. Carat (with a C) is used to measure the weight of precious stones like diamonds, emeralds and rubies. However Karat (with a K)  is used when referring to gold. It is the amount of gold alloy out of 24 parts. Meaning 22k gold is 92% pure gold with 8% being a combination of other alloys or impurities mixed into the gold, such as silver or copper. 

The fineness of gold represents the weight of gold, in proportion to the total weight which includes any other base metals and impurities, in parts per 1000. 

Bering Sea Gold Purity

Placer gold is more pure than lode gold

Lode gold is gold that is found in veins in rocks. This is the type of gold that most inland commercial mines are after these days. There are various types of lode deposits that require specialized methods to extract and mine but at its simplest form, lode gold is the veins of gold found in rocks that are the source of placer gold.

Placer gold, the type that Bering Sea dredges recover, is gold that has eroded or broken off from the main lode vein and traveled down mountains, in waterways, with the heavy gold settling on the bottom of streams, lakes, rivers or the sea floor. This is the type of gold most miners are familiar with, the stuff you can find with a gold pan.

Find placer gold deposits

As it travels impurities and corrosive metals such as copper, quartz or silver are eroded away, making placer gold on average much purer than lode gold.

However, due to the soft nature of gold, other impurities work their way into the gold under the surface area, which means it’s never 100% pure.

Natural gold purity ranges are extremely broad

With placer gold, you never know what you’re getting. Placer gold can be anywhere from 50% gold up to about 95% gold depending on the minerals in that occurrence. Placer gold often has silver, copper and other metals mixed in with the gold at some percentage.

Professional Prospector Dan HurdHow to test gold for its purity. *And give it a value* (Youtube)

Overall, the percentage of pure gold in a placer deposit is a tricky thing to pin down as it can fluctuate dramatically in a small area.

It can be as low as 50% pure or as high as 95% pure.

Most gold is somewhere between those extremes. US natural gold purity is typically cited between 70% and 85%.

Now let’s look at Alaska, since that’s where we recover gold.

A 1981 report by the University of Alaska Fairbanks examined gold from 550 separate creeks all over the state that produced >100 ounces of gold between 1900 and 1974 and came to the conclusion that, on average, the state produces gold between 88% and 91% pure at the region level. That’s a pretty stable average over a broad area.

The Seward Peninsula region, which has 9 mining districts including the Nome District where we dredge for gold, has a mean fineness of 908, or 90.8%, the highest out of all those regions. 

Purity of Alaskan Placer Gold

But as you break it down into individual districts and creeks you’ll find that some are much better than others.

The Nome District as a whole has a mean fineness of 908 with the beach and sea floor off the beach having a mean fineness of 918, proving that Bering Sea Gold is some of the purest around, averaging over 22k!

That makes some sense because the gold in the Bering Sea has traveled the furthest and had the most impurities eroded away compared to gold closer to the source.

If you acquired some placer gold from somewhere else in Alaska, say Utopia Creek in the Koyukuk region, your placer gold would only have a fineness of 734, or a 73.4% purity that puts it between 16k and 18k.

Some places, like Australia, are famous for having nuggets with extremely high gold purities. But there are very pure nuggets found outside Australia, and plenty of lower quality nuggets found within the country.

So how can you know? Besides looking for a report about your area similar to the Alaska study above, you can test it!

How can I test the purity of my gold?

First thing you can do is look at it. The deeper the yellow-orange color, the more pure your gold is. Compare it to gold bullion if you have any lying around.

A more reddish color means it has more copper and a whitish color means it has more silver.

Acid Test

Another option is to perform an “acid test” on your gold.

The short explanation is you scratch your gold on a dark slate and use acids to see if it dissolves. The stronger the acid required to dissolve your gold, the higher the purity. It’s cheap and simple, but it’s a bit of a process and doesn’t give you much precision if that’s what you want.

Dan Hurd has a great video about testing gold with an acid test.

Get an Assay Done by a Refining Company

The acid test has limits. For one it just gives you an estimation within a couple of karats. Another is that it just tests the outside of a piece of gold. A gold nugget could have larger chunks of impurities hidden inside.

In our case, the biggest limit is the acid test is most useful for testing large gold nuggets, ore, or jewelry. It’s not always helpful for fine gold, because the impurities are often small pebbles mixed in with our gold. When you’re cleaning up hundreds of ounces a summer you can’t pan out every single speck of heavy mineral or black sand. It would take too much time!

So when we want to find both the purity of the actual gold and impurity melt loss in order to save our placer gold for our paydirt bags, we’ll send a sample in to a professional refiner for an assay.

Historically we’ve used Oxford Assaying and Refiners, but there are several reputable gold refiners (and several you should avoid). Here’s an example of the report we got back from Oxford in 2020.

As you can see, this one told us our sample was almost 89% pure.

We can send in a sample and know to the 100th of a % how much gold, silver, and other impurities are in our natural gold!

We like using a professional refiner because it’s easy, convenient, and completely accurate. They have advanced chemical processes they use to get a very accurate purity reading.

Know if you send and sell gold to a refiner they’re going to take a % of the metal or cash as a payment for their services, so it can be a lot more expensive than a simple at-home acid test. But if you want a truly accurate read, especially if you have a lot of gold, it’s our favorite option.

Don’t melt down your gold nugget to remove impurities!

Maybe you have a gold nugget and you want to determine the purity. Great!

In my opinion the acid test is your best bet. You won’t get a perfectly accurate read on impurities in the nugget, but you don’t want to melt it down either because you’re losing the value and the beauty of the nugget!

Even a nugget as small as a few grams is actually more valuable than the weight of the gold melted down because the intrinsic beauty, uniqueness, and rarity gives it some extra value.

So don’t melt those nuggets down!

Do you know the purity of your placer gold?

Let me ask you a tough question – do you know the purity of your placer?

If you love to pan paydirt or invest in natural gold, you may think you’re getting a good deal, but you might be paying a secret tax – a big one – in impurities. There’s just no way to know unless you know where your gold comes from.

Just like the USDA says Know Your Farmer, Know Your Food we like to say Know Your Miner, Know Your Gold.

All of our gold at Bering Sea Paydirt comes straight from the Bering Sea. You can even watch us recover it it on Bering Sea Gold on Discovery Channel. 

If you want to prospect alongside us at home consider picking up a bag of our paydirt.

All of our gold at Bering Sea Paydirt comes straight from the Bering Sea. You can even watch us recover it it on Bering Sea Gold on Discovery Channel. 

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