When reading online about gold, there are numerous terms such as ounces, fineness, karats, ppm and ppb, which may be confusing at first. In this post I explain the meaning for these terms from a prospector and geological perspective.
What Is an Ounce of Gold?
When people talk about “ounces” of gold, they refer to the weight of the gold itself measured in troy ounces (“oz”). The use of troy ounces to measure the weight of gold has been used for centuries and is widely established around the world.
One troy ounce is equivalent to 31.1034768 grams
So if you find a gold nugget that weighs 100 grams, simply divide by 31.1034768 to obtain its weight in ounces:
(100 g) / (31.1034768 g/oz) = 3.21507 oz
Troy ounces (the standard in measurement of precious metals such as gold, silver and platinum) must not to be confused with “regular” or avoirdupois ounces (equivalent to 28.34 grams or 437.5 grains). A troy ounce is heavier than a standard avoirdupois ounce. In this website, when I refer to “ounces”, I mean “troy” ounces.
It is believed that the troy ounces were first used in the Middle Ages at Troyes in northeastern France, where a troy ounce was the weight equivalent of 480 grains, or 20 pennyweight. A grain weighs 64.79891 mg.
The standard gold bar held by central banks and traded by bullion dealers is the 400 troy ounce (12.4 kg), known as the “Good Delivery Bar”. The minimum purity required on “Good Delivery” bars is 99.5%
Although the troy ounce weight is the most commonly established around the world, there are other systems to weigh gold used in different countries:
- Tael: it
is used in China, and is equivalent to 37.429 g or 1.2034 troy ounces;
- Tola: used
in India and other parts of the Middle East; it is equivalent to 11.6638 g or
0.375 troy ounces
- Baht: used
in Thailand, it is equivalent to 14.62 g or 0.47 troy ounces.
- Chi: used
in Vietnam, it is equivalent to 3.75 g or 0.1206 troy ounces. Also common in
Vietnam is the Cay or Luong, equivalent to 37.50 g or 1.206
troy ounces.
- Don: used
in South Korea. Like the Chi in Vietnam, it is equivalent to 3.75 g or 0.1206
troy ounces.
- Meshgal: used in Iran, it is equivalent to 4.6083 g or 0.1482 troy ounces
The metric system (e.g. kg, or decimal equivalents such as tons or grams) are also frequently used, particularly in the mining industry. One metric ton of gold, or 1,000 kg of gold, is equivalent to 32,150.7 troy ounces.
What is a troy ounce of gold worth today?
To find out the current price of gold, refer to the chart below (in US$), updated daily:
How is the Purity of Gold Measured?
Gold found in nature is commonly alloyed with other metals such as silver, platinum, etc. Gold fineness is a common term used to describe the gold’s purity or the composition of gold alloys.
Briefly, fineness refers to the proportion of gold within an alloy of impure gold and is expressed in parts per thousand (i.e., a fraction of 1000).
For example, a gold nugget that contains 85% gold and 15% silver is referred to as “850 fine”.
When gold contains 100% gold, it is referred to as “fine gold” or commercially “pure” (i.e., “1000 fine”).
Similarly, gold karats also describe the purity of gold. However, gold karats are measured in portions of 24.
For example, 24-karat gold is 100% gold, or 1000 fine, but a 12-karat gold means that the alloy contains only 50% gold, i.e. it is 500 fine. Note that a 12-karat gold alloy (500 fine) means that it will contain 50% of another metal (or a combination of metals).
The term karat began being used in ancient Asian bazaars, where carob seeds were used in balance scales to measure the weight of gold. Hence, karat comes from the word carob.
Karats are often confused with carats (also known as metric carats). A carat is a measure of weight equivalent to 200 mg, and is used to measure gemstones, diamonds, pearls, etc. That is, a one carat diamond means that is weighs 200 mg.
Gold Alloys Commonly Used in Jewelry
As gold is too soft for jewelry, it is intentionally alloyed with other metals, which reduces the purity of gold. However, the alloy increases the strength and malleability of gold, and slightly modifies the natural color of gold.
For example, common yellow gold used in jewellery is 22-karat (22K) and is composed of 91.67% gold, 5% silver, 2% copper, and 1.33% zinc.
White gold, for example, contains 75% gold (18-karat gold), and 25% platinum or palladium. However, a very similar color can be obtained by alloying 75% gold (18-karat gold) with 10% palladium, 10% nickel, and 5% zinc.
Rose gold, a very popular color for gold these days, is made by alloying 75% gold (18-karat gold) with 22.25% copper and 2.75% silver.
Other Units of Measure
In the exploration industry, gold is commonly measured in “parts per million” (ppm). This is the way that assays are reported from analytical laboratories.
One ppm of gold is equivalent to one gram per ton (g/t), i.e. one gram of gold per metric ton of rock. This means that it is necessary to mine, crush and leach one ton of rock to obtain one gram of gold.
Quite often, ppm and g/t are used interchangeably, i.e. they mean exactly the same thing. However, in gold mining g/t is more commonly used than ppm, because most gold mines contain 0.4 to 4 grams of gold per metric ton of rock mined or greater.
However, some mines have gold grades as high as 30 g/t, and sometimes gold can be exceptionally concentrated within small portions of these deposits (>1000 g/t), known as “bonanzas” or “mother lode”.
Some metals such as silver, bismuth, tellurium, copper, lead, zinc, arsenic, mercury, tungsten, and selenium are known as “pathfinder” metals, and these are commonly measured in ppm.
These metals are commonly associated with gold to different degrees, depending on numerous parameters such as depth of emplacement of the mineralisation, fluid source, distance from the source, geochemistry of the host rock, etc.
Modern exploration techniques take advantage of soil programmes and advanced analytical methods to detect “haloes” of such metals in ppm amounts that could point in the direction of the gold source.
In the exploration industry, gold is also commonly expressed as “parts per billion” (ppb). One ppb is equivalent to 0.001 ppm, or 0.001 g/t.
A ppb is a very small unit of measurement and is particularly useful in early stages of modern exploration. It is commonly used in soil sampling programmes to detect minuscule gold anomalies, which could lead to a significant finding beneath surface.
Hence, gold and the pathfinder metals are commonly reported by laboratories either in ppm or ppb, depending on the metal and on the analytical method used.