9ct vs 18ct Gold: What's the Difference?

When people ask me about the difference between 9ct and 18ct gold, they usually expect a simple answer, but the real answer is slightly more interesting than that.

The numbers refer to purity

Carat is a measure of how much of the metal is pure gold. Pure gold is 24 carat. 18ct gold is 18 parts gold out of 24 - 75% pure. 9ct gold is 9 parts gold out of 24 - 37.5% pure. The rest in both cases is an alloy, typically silver, copper, and zinc in varying proportions depending on the maker and the colour of gold being produced.

In the UK, 9ct is the minimum that can legally be hallmarked and sold as gold jewellery. Both 9ct and 18ct are genuine gold. One just has more of it.

Colour

Both look gold. Side by side, 18ct is slightly warmer and richer in colour - it has more gold in it, so the colour is closer to pure gold's deep yellow. 9ct is a little paler and cooler. The difference is subtle enough that most people wouldn't notice on a single piece, but if you put them next to each other you can see it.

Rose gold and white gold exist in both carats. The colour in those cases comes from the alloy composition rather than the gold itself - rose gold gets its warmth from a higher copper content, white gold from palladium or nickel.

How they wear

This is where it gets more nuanced. Because 18ct has more gold and less alloy, it's slightly softer. Gold itself is a relatively soft metal - it's the alloy metals that add hardness. So counterintuitively, 9ct gold is harder and more scratch-resistant than 18ct.

For pieces that take a lot of daily wear and contact, rings especially, 9ct holds up very well. For earrings or pendants that don't get knocked around much, 18ct's softness is not a problem and the richer colour is a genuine bonus.

Both will develop a patina over time. Both can be repolished. Neither will discolour or tarnish.

Price

18ct costs more. It contains roughly twice the amount of gold by weight. For a piece of equivalent weight and design, you'd expect to pay significantly more in 18ct. How much more depends on the current gold price and the maker's pricing structure, but as a rough guide you're looking at a substantial difference for heavier pieces, less so for very fine or lightweight work.

9ct is not a compromise. It's a different specification. A lot of the finest British jewellery has been made in 9ct for generations, it's the traditional UK standard and it wears beautifully.

Which should you choose?

For everyday rings, 9ct is a sensible choice - harder, more affordable, and it still looks unmistakably gold. For earrings, pendants, and pieces you want to feel slightly more luxurious in the hand, 18ct is worth considering if the budget allows.

I work in both. If you're not sure which is right for a specific piece, ask me - I'm happy to talk through it.