How small can we see? Pretty small, it turns out. We can see atoms (see above)--not with light but with electrons (light is too crude of a yardstick and can't "measure-down" to the job). In Scanning Electron Tunneling Microscopy (STM), a tiny metal wand just an atom or two thick approaches a surface. Electrons, dripping from the tip of the sweeping probe, jump to a surface below, feeding signal back and mapping the atomic topography:
Successive traces of electronic signals become a "photo" of the surface. The way STM works reminds me of the spark of life implied so long ago here. The technique is more fully explained here.
Suppose we could zoom a microscope down onto a pure copper surface to find out what's really there. I mean really there there. We'd find, even for ultra pure copper--heterogeneity. What looks the same is really different. No matter where we look, about one in three copper atoms has two "extra" neutrons because native copper comes in two isotopic flavors: 63Cu and 65Cu.
A while back, Michael Haz mentioned that pure copper native to Upper Michigan was distinguishable from pure copper native to South America. It's true. Copper sources have isotopic signatures. The natural ratio of the two copper isotopes varies slightly from place to place for various reasons. The reason(s) why they vary are complex and altogether unimportant here. The point is that they differ and they do so in a way that can be reliably measured--like fingerprints. A similar isotopic method has been used to trace the influx of South American silver into European coinage: link