Nothing here yet.
A good birthstone necklace reads as a proper stone, something with a bit of depth and colour to it, sitting on a chain that doesn't go thin and bendy after a fortnight. A poor one reads as a bead of coloured glass on a cheap setting, flat and lifeless from across a room, the sort of thing that looks like a vending machine prize rather than something chosen for someone.
Where it falls short is if you want it to double as your everyday, do everything piece. A birthstone necklace is specific by design, it's tied to a month, a person, sometimes a whole story behind the gift, and that makes it less flexible than a plain pendant. If you're after one necklace that works with absolutely everything in your collection and never looks out of place, this isn't quite that, it's got a job to do and it does that job rather than being neutral background jewellery. The stones that are easiest to live with day to day tend to be the smaller, quieter shades, deep garnet reds, soft amethyst purples, pale aquamarines, rather than anything bright enough to pull focus on its own. Clear stones like diamond or white topaz settings are the most forgiving of all, because they behave like any other clear stone necklace and slot in without announcing themselves.
Where it gets tricky is layering. Birthstone necklaces often come with a small claw setting or a halo of tiny accent stones, and that detail can look fussy stacked against anything else with texture, so chunky chain necklaces or anything with its own obvious design tend to compete rather than complement. Worn on its own, or with one plain fine chain behind it, it looks considered. Worn with three other statement pieces, the stone gets lost and the whole thing looks cluttered rather than personal.
How it ages depends entirely on the setting rather than the stone. Real gemstones don't fade, but plated prongs holding them in place can go dull or slightly bronze after months of daily wear, hand washing, bag straps catching on the chain, the usual. Rinse it occasionally, keep it away from perfume and chlorine, and store it flat rather than tangled with everything else, since the setting is usually the first thing to show wear, not the stone itself. A chain that's gone thin or kinked after a few months of actual wear is telling you something about the quality you didn't notice in the shop.
Tomorrow morning I'd wear it with a plain jumper, nothing else at the neck, and let the stone be the only thing doing any talking.