The counter's trade-off, not a grade of quality

At the counter, the choice of karat mark is a choice between durability, colour and gold content. A lower mark is harder and more resistant to wear, but it contains less gold. A higher mark is softer and richer in colour, but it contains more gold. The published price can speak to the gold content, but it cannot speak to durability or colour. The buyer must weigh those with no help from the price tag.

What the mark does to the metal

Pure gold is soft. It bends and scratches easily. To make jewellery that will hold a stone or survive daily wear, other metals are added. The karat mark states how much gold is in the alloy, and the rest is those other metals. The lower the mark, the harder the metal tends to be.

That hardness is a real advantage. A ring worn every day, on a hand that opens doors and lifts bags, will show wear. A lower karat mark will show less of it. The trade-off is gold content: the lower the mark, the less gold is in the piece, and the less gold value it carries.

Colour and the mark

The metals added to gold change its colour. Copper gives a redder tone, silver a greener one, and palladium or nickel a whiter one. The karat mark affects the colour because it determines how much room there is for those other metals. A 24 karat piece is a rich yellow, because there is almost nothing else in it. An 18 karat piece can be yellow, white or rose, depending on the alloy.

A buyer who wants a particular colour may have to accept a lower mark. White gold, for example, is almost always 18 karat or lower, because the metals that make it white also make it hard. The published price says nothing about colour; it only says how much gold is in the gram.

Where durability and gold content pull apart

The two main considerations at the counter pull in opposite directions. A piece that will be worn hard needs a lower mark for durability. A piece bought for its gold value needs a higher mark. The buyer cannot have both maximum hardness and maximum gold content in the same piece.

The published price makes the gold content visible. The buyer can see exactly how much less gold is in an 18 karat piece compared to a 22 karat piece. But the price says nothing about how much harder the 18 karat piece is. That difference must be judged by hand and by reputation, not by arithmetic.

What a published price can and cannot speak to

A published price per gram at each mark can speak to one thing: the gold content. Because the mark states the proportion of gold, the price per gram can be divided by that proportion to give a price per gram of fine gold. That division shows the tariff structure and lets the buyer compare marks on equal terms.

The published price cannot speak to durability. It cannot speak to colour. It cannot speak to the skill of the maker or the design of the piece. Those are qualities that vary from piece to piece and shop to shop, and no daily price list can capture them.

The buyer's actual decision

At the counter, the buyer looks at the piece, tries it on, and asks about the mark. The seller may explain the trade-offs, or may not. The buyer who has read the arithmetic knows that the lower mark is not a cheaper market, but a dearer tariff on gold. The buyer who has worn jewellery knows that the lower mark may last longer.

The decision is personal. A wedding ring worn every day may be 18 karat for durability, even though the gold content is lower. A bangle bought as an investment may be 24 karat, even though it will scratch. The published price informs the gold half of the decision; the rest is in the hand and the eye.

What the seller may not say

Sellers often describe a lower mark as more durable, which is true, and sometimes as better value, which is not. The arithmetic shows that the lower mark costs more per gram of fine gold. The seller may also describe a higher mark as pure and therefore better, without mentioning that it is soft.

The buyer should treat the seller's words as one input, not as advice. The hallmark is the legal statement of gold content. The published price is the public suggestion. The rest is judgement, and the buyer brings that to the counter.