XRF Testing
A non-destructive X-ray test that reads metal purity in seconds. What the karat meter in a jeweller's shop actually is.
X-ray fluorescence excites the atoms in a metal and reads the energy they give back, which identifies each element and its proportion.
It takes seconds, marks nothing, and is what assaying centres and gold buyers use for a first reading.
Its limitation is depth. XRF reads only the surface layer, so a heavily gold-plated base metal can give a misleadingly good result.
What you will hear
“The karat meter proves a piece is solid gold.”
In fact: It proves the surface is. A thick plating over brass can pass an XRF check, which is why fire assay still exists for anything high in value.
When you are buying
When selling old gold, ask to watch the XRF reading and see the screen. If a buyer files or cuts the piece instead, that is a deeper test and reasonable, but it should be agreed first.
Assay
The laboratory test that establishes how much gold is actually in a piece.
Hallmark
An official stamp certifying precious metal purity. In India, BIS hallmarking is mandatory for gold jewellery.
Fineness
Purity expressed in parts per thousand rather than karats. 999, 916, 750 and 585 are the numbers you will see stamped.
Gold Plating
A thin layer of gold deposited over a base metal. Gives the colour of gold at a fraction of the cost, and wears off over time.
X-Ray Testing
The only reliable way to tell a natural pearl from a cultured one, by imaging what is inside it.