What it means
An assay turns a piece of metal into a verified fact. The test reports how much gold, silver, platinum or other valuable content a sample holds, usually as a fineness for bars or a grade per tonne for ore.
Methods vary with the question being asked. Fire assay, the centuries-old standard for gold, melts the sample with fluxes to separate the precious metal and weigh it.
Modern laboratories add instrumental techniques such as spectrometry for speed and multi-element detail. In bullion markets the assay underpins trust.
Bars from accredited refiners carry stamps of weight and fineness backed by the refiner's assays, and good-delivery rules from bodies such as the London Bullion Market Association let those bars trade globally without retesting at every sale. In mining, assays are the economics.
Exploration companies drill core samples and send them to independent labs, and the reported grades decide whether a hole is a discovery or a disappointment. Ore grade multiplied by tonnage is the deposit.
That power invites fraud, and the industry guards the chain of custody. Reputable programs use certified reference materials, duplicate samples and umpire assays at second laboratories, because a salted sample once cost investors billions in scandals such as Bre-X in 1997.
For an investor reading a drill press release, the assay table is the only number that matters. Headline grades, interval lengths and the lab's accreditation tell you whether the announcement is substance or promotion.
Coin and jewelry buyers meet the assay as a hallmark. Official assay offices in countries such as the United Kingdom test and stamp articles to guarantee fineness, a consumer protection system centuries old.
The United States Geological Survey publishes reference data on analytical methods for ores and minerals, and the LBMA sets the good-delivery standards that make refiner assays acceptable in the global bullion market. Knowing who tested the metal, and how, is the difference between evidence and advertising.
In practice
Real-world examples.
Example
A one-kilogram gold bar stamped 999.9 by an LBMA-accredited refiner trades in Zurich without retesting because the refiner's assay is trusted under good-delivery rules. The buyer pays on the stamped weight and fineness, which saves days of testing and the cost of melting a sample.
Example
An exploration program sends every tenth sample for umpire assay at a second laboratory, catching a calibration drift before it corrupts the resource estimate. The company corrects the first laboratory's results and avoids publishing grades that later had to be withdrawn.
Example
A UK silver candlestick bears a London assay office hallmark, certifying that independent testers verified its 925 sterling fineness before sale. The buyer can rely on that stamp as evidence of purity, without needing to test the metal personally.
Formula
Calculation
There is no single formula. The working metrics: fineness = pure metal weight / total item weight, expressed in parts per thousand, so 999.9 fine gold is 99.99% pure. For ore, contained metal = tonnage x grade; 2 million tonnes grading 3 grams per tonne holds about 6 million grams, roughly 193,000 troy ounces, of gold.
Worked example for a bar. A one-kilogram bar assayed at 999.9 fineness holds 1,000 g x 0.9999 = 999.9 g of pure gold. One troy ounce is about 31.1 g, so the bar contains about 999.9 / 31.1 = 32.2 ounces. At an assumed price of $2,000 an ounce, the pure gold content is worth about 32.2 x $2,000 = $64,400. A bar assayed at only 900 fineness would hold 900 g, about 28.9 ounces and $57,800 at the same assumed price, which shows why the assay result drives value.Case study
Seen in the real world.
This case study is fictional and illustrative. A junior miner announces a drill intercept of 25 metres at 4 grams of gold per tonne from an accredited lab, with duplicate assays matching within tolerance. An analyst multiplies grade by modelled tonnage, sees a viable deposit at current prices, and initiates coverage; a rival's unverified 'visual estimate' release the same week is ignored. The analyst's note lists the checks she made before trusting the number.
She confirmed the laboratory's accreditation, read the intercept table for the length of each interval, and checked that standards and blanks had been inserted among the samples to catch contamination or drift. In this fictional story the junior miner's share price rises on the news, and a later round of drilling confirms the grade over a wider area. The illustrative lesson is that the market rewarded the announcement with the better evidence, not the louder one.
Watch out
Common mistakes.
- Trusting headline grades without checking intervals; a spectacular grade over a tiny width can be worth less than a moderate grade over tens of meters. Multiply grade by thickness before valuing any intercept.
- Ignoring the laboratory and chain of custody; assays from unaccredited labs or promotional 'grab samples' prove little. Look for independent accredited laboratories, duplicates and certified reference materials.
- Assuming hallmarks equal assays today; a hallmark certifies fineness at testing, but wear, repair and forgery happen. High-value bullion purchases still warrant verification of weight and purity.
Questions
People also ask.
What is an assay?
It is a laboratory test that determines the composition and purity of a metal or ore sample. Results are expressed as fineness for refined metal or grade per tonne for ore, and they set what the material is worth.
Why do assays matter in mining?
They turn drill samples into ore grades, and grade times tonnage defines the deposit's value. Exploration announcements, resource estimates and mine economics all stand on assay results, which is why labs and custody chains are scrutinized.
What is a hallmark in relation to an assay?
A hallmark is the stamp an official assay office applies after independently testing a gold, silver or platinum article. It certifies the item's fineness and origin, giving buyers a state-backed guarantee of purity.
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