Found these shiny discs at a thrift store, what are they exactly?

Those are the real deal, most likely reject or surplus wafers that got sold off as scrap. Each one is a single crystal of silicon that a fab grew, sliced, lapped, and polished, then ran thru dozens of masking steps to print the same chip over and over in that grid you're seeing. Before the wafer ever gets cut apart, an automated tester probes every single chip on the surface. If a wafer comes back with too many failures, or the whole lot gets cancelled or fails qualification, the thing goes in the scrap bin whole. Scrap gets sold off by weight to surplus dealers, then gets passed down the chain untill somebody's box ends up at a secondhand shop. Your chipped one was probly allready doomed anyway, so don't sweat the sticker too much, though for the others, peel them off with isopropyl alcohol and a cotton swab rather than your fingernail. The patterned plating scratches and flakes easier than you'd think. The numbers along the edge are a laser-scribed ID, usally lot number, wafer number, and date, so the factory could trace it back if a customer complained. Dull side is the back, where they grind it flat. Shiny side is the circuit side. For display, shiny side up, always. The faint rainbow colors you see at an angle are light diffracting off the circuit layers, and they change depending on where you stand, which is half the fun. Lay it flat on a piece of felt or dark cloth and don't let anything metal near it. They're also surprisingly sharp once chipped, more like glass than plastic, so handle a broken one by the edges and keep it away from kids.

If you want more than a shelf decoration, grab a cheap jeweler's loupe and look at individual chips. Some are actual product designs, some are just test patterns, and figuring out which is which is a good evening. People also cleave scrap wafers into small tiles for jewelry or pendants, since they snap cleanly along crystal planes if you scribe them first.

Funny reading this because my uncle worked at a semiconductor plant years back and I ended up with a couple of these in a drawer of his old stuff. Never knew what the edge markings were until now, always assumed they were just serial numbers. Mine have that same rainbow shimmer when you tilt them under a lamp. Kind of a side question but does the grid layout tell you anything about what the chips were for? Like if the squares are huge versus tiny, does that mean an older or simpler part? Mine look way coarser than the photos I've seen of modern ones and now I'm wondering if I've got something ancient sitting in that drawer.

Not every scrap wafer went through all those masking steps though. Fabs run plain monitor wafers through the line regularly just to check the process is behaving, and those come out blank, no pattern at all, nothing worth dicing. They get scrapped by the barrel load. So if someone finds a shiny disc with no grid on it, that doesn't mean it's fake, it might just be one of those. The patterned ones with the little squares are the more interesting find, but the blanks are common as dirt in the surplus stream. Check under a loupe before assuming a dull looking disc is a dud.

Update: the tip about tilting them under light helped, caught the rainbow shimmer on a couple of mine that I'd written off as blank, so now I'm wondering if I actually have one of those plain process-check discs in the box. Still stuck on the edge markings though. Is there any way to read them to figure out what was printed on the die, or is that info basicly lost?