9ct gold can become dull, discoloured or develop surface tarnish because most of the alloy is made from metals other than pure gold. That does not automatically mean the item is fake.
Why 9ct gold can change colour
UK 9ct gold is 375 fineness: 37.5% gold, with the remainder made from alloying metals chosen to influence colour, hardness and working properties. Those other metals can react with moisture, air, cosmetics and chemicals more readily than pure gold.
Tarnish versus worn plating
On a solid 9ct item, surface discolouration can often be cleaned without revealing a different base metal underneath. A plated item is different: once its thin gold layer wears through, the exposed base metal may create a permanent colour contrast unless the piece is replated.
What to check before cleaning
- Confirm whether the piece is solid gold, plated, vermeil or another finish.
- Look for a recognised hallmark where UK hallmarking rules apply.
- Be cautious around gemstones, pearls, glued settings and deliberately oxidised details.
- Avoid aggressive abrasives that remove metal or surface coatings.
How to reduce discolouration
Remove jewellery before swimming or using strong cleaning chemicals, wipe it after heavy perspiration, and store pieces dry and separately. For valuable or delicate jewellery, professional cleaning is the safer route when the construction is uncertain.
Buying takeaway: Do not judge authenticity from colour alone. Combine material description, hallmarking, weight and seller information.