Jewellery is a difficult gift because it is worn on the body and because taste in it is narrow. The good news is that most of the guesswork can be removed by paying attention for a week before you buy.
Look at what they already wear
If everything they own is yellow gold, a white gold piece will sit in a drawer. If they wear one thin chain and nothing else, a heavy statement necklace is the wrong gift however beautiful it is. Existing jewellery is a stated preference, not a guess.
Buy for the life, not the photograph
Studs get worn many times more often than drops. A bezel setting survives a commute; a high solitaire snags on a scarf. Ask where the piece will actually be worn before choosing how dramatic it should be.
Avoid the sizing problem
If you do not know a ring size and cannot find out without ruining the surprise, a pendant, a bracelet or a pair of earrings removes the risk entirely. Rings can be resized afterwards, but that means a second trip and a delay.
When you genuinely cannot decide
A birthstone piece is the safest personal choice, because the stone is chosen for them rather than by you. It reads as thought rather than as taste, which is usually what a gift is trying to say in the first place.
