Assay
The laboratory test that establishes how much gold is actually in a piece.
Assaying is done by fire assay, which is destructive and definitive, or by X-ray fluorescence, which is quick and non-destructive but reads only the surface.
Because XRF reads the surface, a heavily plated base-metal item can briefly look like gold to it. Fire assay cannot be fooled that way.
In India, assaying and hallmarking centres are licensed by BIS and are separate businesses from the jewellers who send them work.
When you are buying
If you are buying a high-value unhallmarked or inherited piece, pay for an independent assay before agreeing a price. It costs very little against the value of what is being tested.
Hallmark
An official stamp certifying precious metal purity. In India, BIS hallmarking is mandatory for gold jewellery.
HUID
The six-character code stamped on every hallmarked Indian gold item, unique to that single piece.
Fineness
Purity expressed in parts per thousand rather than karats. 999, 916, 750 and 585 are the numbers you will see stamped.
Valuation
An independent written assessment of what a piece is worth, usually for insurance rather than for sale.