A hallmark in a piece of jewellery is a small engraved code that typically tells you what the jewellery is made of and who is behind it. The most common one is a fineness number - three digits such as 585, 750 or 925 - that indicates how large a proportion of pure precious metal the jewellery contains. In addition to the fineness, there may be a maker's mark that identifies the manufacturer or retailer, and in some countries an official assay mark that confirms the purity of the metal. The hallmarks are small for the simple reason that they need to be almost invisible in the finished piece.

You will find them on the flat, hidden surfaces: the inside of a ring shank, the back of a pendant or an earring, or on the clasp of a necklace or bracelet. They are often so small that they are hard to read with the naked eye, and a loupe or a good magnifying glass makes them easier to decipher.

Fineness numbers on gold and white gold explained

Fineness is a measure of how pure the precious metal is. Pure gold is too soft for jewellery that is worn daily, so it is mixed with other metals to form an alloy that is stronger and more hard-wearing. The fineness number tells you how much pure gold the alloy contains, expressed in parts per thousand.

585 means that the jewellery consists of 585 thousandths pure gold, that is 58.5 percent - this corresponds to 14 carat. 750 means 75 percent pure gold and corresponds to 18 carat. The higher the number, the greater the proportion of pure gold.

Here you need to keep two things apart, which unfortunately are spelled almost the same. Karat with a k is a measure of the purity of the gold, while carat with a c is a unit of weight for diamonds and other gemstones. A hallmark with "14K" or "585" is therefore about the gold, not about the stones in the jewellery.

White gold is hallmarked according to the same principle as gold, because it is the same alloy with the same content of pure gold - the difference lies in which metals the gold is mixed with, and in the surface treatment. A piece of white gold jewellery can therefore bear a 585 or 750 hallmark in exactly the same way as a piece of gold jewellery. At Longvé, 14K gold and 14K white gold are the core metals, and both correspond to a 585 hallmark. If you want to see the difference between the two colours in practice, you can compare 14k yellow gold with 14k white gold.

What does 925 mean in a piece of silver jewellery?

925 is the classic hallmark for sterling silver. The number means that the jewellery consists of 92.5 percent pure silver, while the remaining 7.5 percent is typically copper, which makes the metal stronger. Pure silver, like pure gold, is too soft for everyday use, and the alloy provides a hard-wearing material that still has the bright colour of silver.

In addition to 925 you sometimes see the number 800, which indicates a lower silver fineness of 80 percent. 925 is, however, the common standard for jewellery silver. At Longvé, sterling silver is used for, among other things, signet rings and a few necklaces and bracelets.

The word "sterling" or the abbreviation "STG" sometimes appears alongside the number and means the same thing: 92.5 percent silver.

What is a signet ring, and how is it hallmarked?

A signet ring is a ring with a wide, flat plate on top. Historically, the plate was used to engrave a coat of arms or a monogram, so the ring could be pressed into wax and serve as a kind of signature. Today signet rings are worn as jewellery, often without a stone, but some versions have a flat gemstone set in the plate, for example black agate.

The hallmarking of a signet ring itself follows the usual rules for the metal. If the ring is in sterling silver, it bears a 925 hallmark on the inside of the shank, possibly together with a maker's mark. The flat plate on top is intended for engraving, while the fineness hallmark hides inside the ring itself, where it is not visible when the ring is worn.

Maker's mark and assay mark

A fineness number rarely stands alone. Next to it there is often a maker's mark - a small symbol or a combination of letters that identifies the manufacturer or retailer the jewellery comes from. The maker's mark says nothing about the purity of the metal, but about who has made or put their name to the jewellery.

In several countries there is also an official assay mark, which is applied by an independent control body and confirms that the stated fineness number holds true. The rules vary from country to country. In Denmark there is no requirement for an official assay mark in the same way as in, for example, the United Kingdom, but the fineness hallmark and possibly a maker's mark are standard practice for jewellery in precious metal.

If you see several different small hallmarks side by side on the same piece of jewellery, it is therefore typically a combination of fineness and source - and possibly an assay mark, if the jewellery comes from a country where that sort of thing is required.

Are diamonds and gemstones hallmarked too?

No. The hallmarks in a piece of jewellery are about the metal, not about the stones. A diamond's properties - weight in carats, colour and clarity - are not shown by a hallmark in the jewellery, but by any certification or product description that comes with it.

This applies regardless of whether the stone is a natural diamond or a lab-grown diamond. The two types have the same chemical and physical properties, and neither the choice of stone nor the difference between lab-grown and natural diamonds changes how the metal itself is hallmarked. A ring with a lab-grown diamond in 14K gold bears the same 585 hallmark as a corresponding ring with a natural diamond. If you want to see how the two stones look in matching settings, you can compare lab-grown diamonds with natural diamonds.

How to find and read the hallmark

On a ring the hallmark is almost always located on the inside of the shank. If you turn the ring so the inside faces the light, the numbers appear. On an earring or a pendant you need to look at the back, and on a necklace or a bracelet the clasp is the most common place.

Because the hallmarks are so small, a loupe is the best tool. Hold the jewellery up to a good light source and tilt it slightly until the numbers come into sharp focus. If you see a three-digit number, it is the fineness - 585 and 750 point to gold or white gold, 925 to sterling silver. If there are letters or a symbol next to it, that is typically the maker's mark.

If a piece of jewellery has no hallmark at all, it does not necessarily mean that the metal is fake - very fine or very old jewellery may lack a readable hallmark because it has worn away or was never applied. But for newer jewellery in precious metal, the hallmark is the first and quickest indication of what you are dealing with.