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The first thing I check on a silver necklace is the clasp, not the chain. A lovely finish doesn't matter if you're stood in front of a mirror wrestling with a lobster clasp the size of a lentil for five minutes before you've even left the house. Good weight to the chain matters too. If it feels like it might snap if you look at it wrong, it probably will, usually on the one day you've got your hair up and nothing to layer over the gap.
It's not the right choice if you've got skin that reacts to anything other than solid silver, because plated or silver tone pieces can shift colour where they sit against warm skin, especially round the clasp and the bit that rests on your collarbone. It's also not great if you want something that survives being slept in or worn in the shower every day without a second thought. Sterling silver needs a bit of care, a soft cloth now and then, and keeping it away from chlorine and perfume sprayed directly onto skin, which nobody thinks about until the chain's gone a bit cloudy.
In the shop, look closely at the jump rings, the little loops holding pendant to chain, because those are the first thing to open up under normal wear. Give the clasp a proper test, open and close it a few times, not once. Check the chain for any join that looks soldered on wonky, and hold it up to the light to see if the finish looks even all the way along, not just shinier at the parts people touch in the shop.
For shades that are easy to live with, I'd go for a plain polished finish over anything heavily textured or hammered, because the detail in those catches dirt and needs more cleaning to keep looking decent. A slightly warmer, satin finish silver hides small scratches better than a high shine one, which shows every mark within a fortnight of normal wear. Plain chains in a mid weight go with almost everything you already own, a white shirt, a jumper, a vest top in summer, and they layer well if you ever want to add a second one without the two chains tangling into a knot every time you move your head.
One sentence on fit: go for a length that sits an inch or two below where your collarbone meets your neck, since that's the spot that works whether the neckline underneath is high or low.
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