How Pure is Your Gold

When gold isn't always gold

California gold purities

NuggetsAll that glitters is not gold. Indeed, it's probably loaded with silver.

Is gold really gold? Depending on where it's recovered the actual amount of gold, in gold, can vary considerably.

I've known prospectors who could tell you where gold came from by just the look. Someone once showed me some gold flakes recovered from the Feather River and they virtually sparkled in the sunlight due to a high amount of silver. Pretty gold to be sure, but not as valuable as gold from the Yuba River which is a dull yellow.

You can, in fact, tell where gold is from based on what it looks like and this is related to the amount of other minerals mixed with the gold. Different areas produce different gold.

In the 1800's California state geologists spent considerable effort studying gold. As part of this effort research was conducted on the value, or fineness, of gold from different counties and different mines. The fineness of gold is a measure of how pure it is with a value of 100 being pure, or 24 karat (sometimes spelled karat). The karat is a measurement of the percent of gold in a metal. So each karat represents 1/24th of gold. In jewelery gold is often mixed with another metal such as silver or copper. So 18 karat jewelery would be it is 75% gold and 25% some other metal.

The below table provides gold purities by counties in California.

County Average Fineness Fineness
Lode Mines Placer Gold
Nevada 855 645-890 906-935
Placer 792 580-921 940-970
Plumas 851 NA NA
Sierra 858 622-833 948
Calaveras 835 627-885 940-987
Tuolumne 804 NA NA
El Dorado NA 570-901 940-970

Placer and lode gold is rarely pure, but placer gold is almost always purer than lode gold. Lode gold is often mixed with other metals, but once eroded from the source, and tumbled in a river for a few thousands years the other metals will disolve leaving almost pure gold.

"Observations in all parts of the world have shown that placer gold is always finer than gold in the quartz veins from which the placers were derived. The explanation, as has been shown in a most convincing manner by Ross. E. Brown, among others, is that the silver alloyed with the gold is dissolved by the action of surface waters. The purity of the gold becomes greater as the size of the grains diminishes, the explanation being, of course, that the proportionate amount of surface exposed to the action of solutions is greater in the finer gold. An interesting confirmation of this view is recorded by McConnell, who states that examination of nuggets from the Klondike shows taht their surfaces consist of gold of greater fineness than their insides." Waldermar Lindgren, USGS Professional Paper 73.

Silver often gives gold an exceptionally beautiful look. Copper, as you would expect, tends to give gold a greenish hue.

The purest gold in California was mined from the San Giuseppe quartz mine near Sonora with a fineness between 982 to 998, or virtually pure gold. The purest placer gold in California comes from placers near Folsom and ran 974 to 978 in purity. Second to the Folsom area was an area near Oroville which produced placer gold of 922.