You can tell a good birthstone necklace from a poor one before you even get close. The good one catches light properly, the stone sits where it should on the chest and the chain looks like it belongs with it. The poor one has a stone like a bit of boiled sweet on a chain that's already gone thin and dull, and it never quite sits straight.
These aren't for everyone, and I'd say that plainly rather than let you find out the hard way. If you want something that reads as serious jewellery, a birthstone piece with a pale or fussy stone can look more novelty than meaningful, especially if the setting is thin metal with a lot of visible glue. Anyone with a nickel sensitivity needs to check the posts and clasps rather than just the chain, because that's usually where the cheap metal hides. And if you're after one necklace to wear with absolutely everything, a very bright or very dark stone can start to feel like it only goes with certain necklines, which defeats the point a bit. For colours, the ones that live easiest on your neck day to day tend to be the softer, slightly muted versions of a stone rather than the most saturated one in the display, a dusty amethyst over a violent purple, a warm amber over a traffic light orange. They go with more and they don't date the way the brightest version will. Avoid wearing them with other statement necklaces or anything with its own strong colour story, a birthstone piece wants to be the one thing doing the talking, not sharing a neckline with a chunky resin bib or another coloured stone that's fighting it for attention. Over months, what separates the ones that still look good from the ones that look tired is mostly the chain and the setting, not the stone itself. A thin plated chain will often go patchy or slightly green at the clasp within a season, especially if it's worn daily and gets perfume on it, which it will, because you forget it's even on. Stones set with prongs tend to hold up better than glued ones, which can loosen or dull at the edges after a few months in a jewellery pouch at the bottom of a bag. Take it off before you shower or swim, wipe it with a soft cloth after wearing, and store it somewhere it isn't rattling against other pieces, that alone adds a good year or two to how it looks.
Tomorrow morning I'd put mine with a plain crew neck jumper and nothing else at the throat, so the stone has the whole neckline to itself.