The metal is the part that has to survive being worn every day for decades while holding a stone in place. It matters more than it gets credit for.
18k yellow gold
Seventy-five per cent pure gold, alloyed for strength. Warm, traditional, and the colour most people picture when they picture gold. It flatters warmer skin tones and it never needs re-plating, because the colour goes all the way through.
18k white gold
Yellow gold alloyed with white metals, then rhodium-plated for the bright cool finish people expect. It is the most popular choice for solitaires because it disappears and lets the stone do the work.
Worth knowing — The plating wears. Expect to have it re-plated every few years — sooner on a hand that is used hard. This is normal maintenance, not a defect, but nobody enjoys finding it out afterwards.
18k rose gold
Gold alloyed with copper. The copper makes it both the warmest in colour and the hardest of the three golds. Like yellow, the colour is the metal rather than a coating, so it does not need re-plating.
950 platinum
Ninety-five per cent pure, naturally white, and the densest of these metals — a platinum ring is noticeably heavier in the hand. It needs no plating because it is already the colour it will always be, and claws cut in platinum hold a stone more securely than gold claws of the same dimensions, because the metal moves rather than snapping.
Worth knowing — It scratches into a soft grey patina rather than staying mirror-bright. Most people come to prefer it; if you want permanent mirror polish, it can be re-polished, but expect the patina to return.
Silver: what it can and cannot do
Sterling silver is about 2.5 on the Mohs scale against platinum's 4 to 4.5. It bends, it wears thin, it tarnishes against skin, and claws cut in it loosen over years of daily wear, which puts the stone you spent the money on at risk. For the ring that is worn while washing up, carrying shopping and sleeping, for fifty years, we recommend 18k gold or platinum and will say so. We do make 925 silver solitaires on request, with a bezel rather than claws, as a placeholder to propose with, a first ring or an occasional piece, and the stone moves to gold or platinum later. The silver page sets it out honestly.
14k gold: harder, paler, and available
Fourteen carat is 58.3 per cent gold against 18k's 75. It is harder, which matters on a hand that is used hard, and it costs less, which can put the difference into the stone. The colour is visibly paler and less saturated, and the higher proportion of alloy means more people react to it against the skin, so we recommend 18k and make 14k on request for the hands and budgets it suits. BIS hallmarks it 585. The 14k page sets out the trade side by side.