Fineness and karats
999.9 (or 24k) means 99.99% pure gold. 18k is 750 parts per thousand — the standard for fine jewellery in most European markets. 14k (585) dominates in parts of North America and Eastern Europe.
For investment bars, 999 and 999.9 are the accepted standards. Jewellery alloys trade purity for hardness and colour range.
Hallmarking systems
Hallmarking is compulsory in many jurisdictions and certifies both fineness and the identity of the assay office. UK hallmarks carry a date letter; Swiss marks a fineness stamp and control mark; French pieces carry a guarantee mark for imported items.
Recognising the mark of a reputable assay authority is a basic due-diligence step when buying second-hand or across borders.