Imagine a wee tiny grain. It builds up a coating (usually calcium carbonate) like a geologic pearl, forming an ooid. Then have lots & cement them together into a rock.
A: And this is why rock ID from visuals alone is futile! (&/or why you read the museum signs, but it’s Sunday night, that isn’t happening)
Totally diff material & mechanism for same basic aesthetics.
Mineral 1 precipitates then ~something happens~ (like temp increase to get a lil melty) to round edges, then more crystallizing of mineral 2, repeat-repeat-repeat to grow moar rims. Then crystallize matrix.
It’s still coating spheres & gluing them together.
Details: mindat.org/loc-245214.html