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How pure is a gold nugget? Karats of natural gold explained

By Raquel · Updated 19 October 2026 · 4 min read

The gold in a nugget is never 100% pure. Nature almost always mixes it with some silver, and the proportion changes from one deposit to another. Here is how pure nuggets are, how purity is measured and why it matters.

Native gold: almost pure, but not quite

Nuggets are native gold, meaning gold that occurs in nature as a metal rather than combined with other elements, as iron or copper are in their ores. But “native” does not mean “pure”: inside the nugget, gold forms a natural alloy with other metals, mainly silver and, to a lesser extent, copper.

The proportion depends on the conditions in which the gold formed, so each deposit has its own “signature”. It is one of the reasons why the colour of nuggets varies from one region to another.

Karats and fineness: how purity is expressed

Gold purity is expressed in two ways. In karats, out of 24: 24-karat gold is pure gold, and 18-karat gold has 18 parts of gold in every 24. And as fineness, in parts per thousand: 750 fine means 750 parts of gold in every 1,000.

How many karats is a gold nugget?

Most nuggets are between 20 and 22 karats, that is, between 83% and 92% gold. There are differences by origin: Australian nuggets tend to be very pure, often 23 karats or slightly more, while Alaskan nuggets usually sit at the lower end of that range, which is still high-purity gold.

When silver is very abundant, the alloy becomes paler, almost greenish-yellow, and has a name of its own: electrum, the same metal used for some of the very first coins in antiquity.

Colour gives you a clue

As a rule of thumb, the deeper and warmer the yellow, leaning towards orange, the more gold the nugget contains; the paler it is, the more silver. It is only a clue, not a measurement: lighting, a clean surface and any quartz matrix change the look a great deal. To see a nugget properly, it should be clean; we explain how in how to clean a gold nugget.

How the purity of a nugget is measured

• X-ray fluorescence (XRF). The method many jewellers and labs use: fast and harmless to the piece. Its limitation is that it analyses the surface, and in some nuggets the outer layer is slightly richer in gold than the inside.
• Fire assay (cupellation). The reference method and the most precise, but it destroys the sample, so it makes no sense for a collector’s nugget.
• Density. By weighing the nugget in air and in water you can estimate whether it is gold and, roughly, how pure. If the nugget contains quartz, the result is less precise. We explain it in how to tell if a gold nugget is real.

Does purity affect the price?

Yes, but less than you might think. Purity determines how much gold a nugget contains, and therefore its value as metal. But a natural nugget sells above that value because of its shape, rarity, size and origin. Between two nuggets of the same weight, a collector usually looks at shape and appearance before a one-karat difference. We go into detail in how much a gold nugget is worth.

In our auctions we always state the exact weight and origin of each nugget, the details that help most when comparing.

Written by Raquel, Gold Nuggets Minerals

Raquel selects, weighs and photographs every nugget we sell. Gold Nuggets Minerals S.L.U. is a small company in Zaragoza, Spain, specialising in native gold. On Catawiki we have sold more than 120 nuggets, with 100% positive feedback and the Top seller badge.