The two terms get used almost interchangeably, but they mean different things — one is a mark stamped on a piece, the other is the testing process behind it.
Short answer: assay is the testing process that determines a metal's actual purity and composition; a hallmark is the physical mark stamped onto a coin or bar certifying the result of that assay — the assay is the verification, the hallmark is the proof of it stamped onto the piece itself. The terms are related but describe different steps in the same overall process.
Assaying is the technical process of testing a sample (or, for modern methods like XRF, the whole piece non-destructively) to determine its exact metal composition and purity. A refiner assays its own output before certifying a bar's stated fineness; a dealer or buyer might assay a questionable piece to verify a seller's claims independently.
Once a piece has been assayed and its purity confirmed, a hallmark — a stamped symbol, often refiner-specific — gets applied to the piece as a permanent record of that verification. A hallmark is a claim, backed by the assay process behind it, that a specific refiner or mint stands behind this piece's stated purity.
A hallmark alone is only as trustworthy as the assay process and institution behind it — this is exactly why LBMA accreditation matters (see our LBMA Good Delivery guide): it's a system for verifying that a refiner's assay process, and therefore the hallmarks it applies, meet a consistent, monitored standard. A hallmark from an unaccredited or unknown source carries less inherent trust than one from an LBMA-accredited refiner.
On LBMA Good Delivery bars, the hallmark typically includes both a melter's mark (identifying the refiner) and an assayer's mark (sometimes the same institution, sometimes a separate verifying party) — a more specific version of general hallmarking that identifies exactly who performed each role in certifying the bar.
Basic home checks (weight, dimensions, magnet tests for obvious fakes) aren't true assay, but a jeweler or dealer with proper equipment — an XRF scanner, for instance — can perform a real, if non-destructive, assay on a piece you're uncertain about, giving you independent verification beyond whatever hallmark is already stamped on it.
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