Karat measures the purity of gold: 18k gold contains 75% pure gold (750‰), 9k gold only 37.5% (375‰). 18k is richer, more valued and carries the French eagle-head hallmark; 9k is harder and cheaper, common in the UK but marginal in French jewellery. For a piece meant to last, 18-carat gold is the benchmark.
For gold, karat (not to be confused with the diamond carat, a weight) expresses purity on a scale of 24. 24k gold is 99.9% pure — too soft for everyday jewellery. It is alloyed with other metals (copper, silver, palladium) for strength:
18k gold has a deeper, warmer colour, better long-term stability and a higher resale value (twice as much gold as 9k). Its richness also makes it slightly softer: heavily worn pieces can mark.
9k gold is harder (so more scratch-resistant for a much-worn band), cheaper, but paler in tone and more prone to tarnish over time depending on the alloy. Its metal value is low. In France it is rarely offered and sometimes seen as less 'noble'.
In France, each gold fineness has its own guarantee hallmark: the eagle head for 18k (750‰), a scallop shell for 14k, and a clover for 9k (375‰). Gold jewellery sold by a professional must carry this fineness hallmark, often alongside the maker's mark (a lozenge). Ask to see these marks and to find them on the invoice.
No, the opposite: 9k gold, with more alloy metals, is harder and more scratch-resistant. 18k is softer but richer, more stable over time and far more valued.
French jewellery tradition favours 18-carat gold, seen as true fine-jewellery gold. 9k is mostly found in English-speaking countries; in France it stays marginal, mainly on imported entry-level pieces.
Look for the fineness hallmark: eagle head for 18k, clover for 9k. A jeweller should be able to show it to you and state it on the invoice, with the fineness in thousandths (750 or 375).
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