Why vintage pearls reward a careful eye

Pearls are the only gemstone that leaves the mollusc already finished, which is why a vintage strand carries something no new piece can: history. It also carries wear. Every outing stretches the thread, transfers skin oils and perfume into the silk, and works at the knots that hold the whole thing together. A strand that looks lovely under shop lighting can be a few months away from scattering across a pavement. Five minutes of close inspection will tell you whether you are buying a bargain or a restoration project.

Take the strand to a window with natural daylight. Avoid the temptation to judge it against black velvet, which flatters everything from perfect Akoya to plastic beads.

Start with the thread, because it tells you everything

Hold the strand up by one end and let it hang, then look along its length at eye level. A sound necklace sits in a neat, gentle line. If the pearls dangle in loose loops or the spaces between them look uneven, the thread has stretched and it will need restringing before you wear it anywhere.

• Dirt between the pearls. Grey or brown grime packed into the drill holes and around the knots means years of perfume and skin oil. It rarely cleans out entirely.
• Missing knots. Every pearl should be individually knotted on a good strand. Unknotted pearls are one broken thread away from a loss no jeweller can repair.
• Fraying or fuzziness, especially near the clasp where the thread takes the most strain.
• Colour. Thread should match the pearls, usually white, cream or a soft grey. Bright new white thread on creamy antique pearls is a sign of a recent, perhaps hasty, restring.

Hand-knotting a 45 cm strand typically costs somewhere between £35 and £70 in the UK, more for large or graduated pearls. Work that into your offer rather than discovering it later.

Check the clasp and the fittings

Clasps take far more punishment than pearls do, and a tired clasp is the most common cause of a lost necklace. Open and close it several times. A bolt ring should snap shut with a firm spring; a box clasp should click and hold; a fishhook should sit snugly without play. Look for a worn tongue, a bent or stretched hook, or a spring that has gone soft and lets the clasp drift open on its own.

Examine the metal where the thread meets the clasp. You are looking for a small curved tube, called a gimp or French wire, that protects the silk from rubbing against the metal. If it is blackened, crushed or absent, the thread there will fail first. Check that any jump rings are fully closed and that a safety chain, if present, is intact. Original clasps in gold or silver add value; a cheap modern replacement does not, and it is worth asking whether the pearls themselves are original too.

Read the lustre in daylight, not in shop light

Lustre is the sharpness of the light reflected from the pearl's surface, and it is the single best guide to quality. At a window, you should see a crisp, bright highlight with clearly defined edges, almost as though the pearl were a tiny mirror. On really fine pearls you may also notice orient — a soft pink, green or silver shimmer that floats over the body colour as you turn the pearl.

Dull, chalky or milky reflections suggest either low-grade pearls or nacre that has worn thin through decades of wear, and worn nacre cannot be restored. Two quick checks are useful: real pearls feel cool against your lips and then warm to your skin, while glass and plastic beads stay colder or feel instantly warm. Rubbing a pearl gently against the edge of a front tooth gives a faint gritty resistance on real nacre, and slips smoothly across imitation. Be very gentle with antique pearls, and never rub hard enough to mark them.

Look closely at the surface, shape and drill holes

Almost no vintage pearl is flawless, and small natural blemishes are part of the charm, particularly if you are styling pearls with chunky knitwear, tweed or denim. What you want to avoid is damage rather than character. Run a thumbnail lightly across any mark: a natural pit feels smooth-edged, whereas a chip or crack catches the nail. Cracks and peeling nacre will flake further, and no restringing will save them.

Check the edges of the drill holes for chipping, which usually points to a rough restring, and look for pearls with an obvious seam or paint flaking near the hole, which are signs of imitation. If the strand is graduated, ask for the size range in millimetres so you know what you are getting, and hold it up to check the pearls graduate smoothly rather than jumping about.

Ask the right questions, then price it fairly

A few direct questions will settle most doubts. Has the strand been restrung, when, and by whom? Are the pearls graduated or uniform, and what is the size range? What metal is the clasp, and is it hallmarked? Has the necklace been shortened, and are there spare pearls or an extender? If you are buying remotely, ask for a photograph taken in daylight against a plain pale background, not on dark fabric.

Then do the arithmetic: the price of the pearls plus the cost of a restring plus anything the clasp needs. If the total still feels like good value, buy with confidence. Once home, keep pearls away from perfume, hairspray and direct sunlight, wipe them with a soft dry cloth after wearing, and store them flat in a pouch rather than hanging, so the thread is not left under tension between outings.