Gold Nugget Purity Explained
You just found a gold nugget.
First of all, congratulations! Finding a nice gold nugget feels awesome!
As you sit and stare at it over the coming weeks you might start to wonder if it’s actually pure gold? After all, it might have some rocks or quartz in it, or some discoloration.
That doesn’t mean it’s worth less than “spot price.” Quite the opposite, actually. Gold nuggets are valued at a premium based on their size, look and rarity. Not just the gold content.
But you might be curious anyways. So let’s take a closer look at how pure gold is in its natural form.
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 gold nuggets are pure, ‘gold’ found in nature is actually an alloy of gold, silver, and/or copper with gold being the majority metal.”
The alloys can vary so dramatically that gold nuggets from different regions actually look different! The color is affected by the amount of copper and silver that’s mixed in, along with other impurities.
Here’s a simple color spectrum chart for gold nuggets.
When gold is mixed with too much silver or too much copper, it stops being gold and becomes something else.
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.
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-occurring 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.
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: Man Made 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 man made and does not occur naturally, it’s used mostly for jewelry. So we won’t talk about it too much here.
Half-Breed: The Bound Silver-Copper Nugget
Speaking of copper and silver, 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
Gold bullion, the purest gold available on the market, is measured using Millesimal Fineness. That’s a system that measures the purity by parts per thousand, or the percentage of gold. In this system “999” means that your gold is 99.90% pure and other metal constitutes only 0.1%. “999.9” means your gold is 99.99% pure, which means only 0.01% is other metal. Both are considered 24K gold.
Refiners can also get gold a little purer than that by making 999.99 gold that’s 99.999% pure, but it starts to get expensive and those products carry a premium. That’s the purest gold that’s commercially available.
The purest gold ever created had 1 more “9.” The Perth Mint in Australia refined 999.999 gold in 1957, which is also called six nines fine. It’s not commercially available today.
Refining 100% per gold isn’t possible with our current laws of physics. It’s like a perpetual motion machine – many have tried, but none succeed.
Defining purity: karats and fineness, explained
The best known methods for defining the purity of gold is by Karat and Fineness, which we explained above.
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.
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 deposit that contains “nuggets.”
As placer gold travels, impurities and corrosive metals such as copper, quartz or silver are eroded away, making it more “pure” or concentrated 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 nuggets are 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 Hurd – How 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, even 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.
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 gold 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 find 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.
The way most professional miners find out their gold purity is from fire assay, the most accurate method. Our article “How to Test the Purity of Your Gold Nuggets and Placer Gold” will go more into detail about the fire assay process along with 3 more ways you can test your gold’s purity, two of them you can even do at home without any expensive equipment.
Get an Assay Done by a Refining Company
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 – it’s worth more than you think!
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!
So, what’s the purity of my gold nugget?
Now that you know what gold purity means and how it’s determined, you might be wondering how pure your gold nuggets are. Head on over to our article “How to Test the Purity of Gold Nuggets & Placer Gold Dust” to see 4 ways you can test your gold to find out how pure yours is!
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3 thoughts on “How Pure Are Gold Nuggets?”
A small correction but in the UK, and many other places outside of the USA, “carat” is used for both gemstone weight and purity of gold.
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