Trace impurities may not have much impact on the way a salt tastes, but the size and shape of the salt crystals definitely do. Again, not because the salt is any different chemically, but because crystals of varying sizes and shapes have different surface areas, and so dissolve at different rates.
Salt grains come in two basic shapes: granules and flakes. Granules are the shape we’re all used to, the little perfect cubes we all know as “table salt”. If you remember from other discussions on the subject of crystals (fat crystals, starch crystals, ice crystals), crystallization is what happens when molecules of the same type start stacking up upon one another. Given that molecules of sodium chloride are cubical, it’s easy to see why they might naturally want to stack up into cube shapes. In fact, given the right conditions, salt can grow into huge cubical crystals known as halite, which can be anything up to 4 or 5 inches across.
But notice I said given the right conditions, for in order for salt to crystallize into cube-like shapes, the salt water brine that creates them must be very pure and heavily saturated (far more so than sea water). Also, the crystallization must happen fairly quickly. Modern table salt manufacturers achieve the right conditions by using a purified brine (pumped out of flooded underground salt deposits) which is then evaporated in a special vacuum chamber. Without all that groovy gear, everyday table salt would not have anything like its current uniformity.
Which brings us to “flake” salt. Flake salt, as the name implies, is fairly flat in shape. Salt makers achieve the shape either by crushing cube-shaped granules with heavy rollers (Kosher salt), or by allowing fairly impure brine to crystallize randomly. Flakes form because impurities get between the gathering salt molecules, preventing them from forming uniform crystal shapes. Such random shapes are typical of salts that are allowed to crystallize naturally in ocean-side evaporation ponds.
Depending on where the ponds are, how the brine is allowed to evaporate, how the salt is harvested, and whether or not it’s washed and/or processed, the flakes can vary significantly. So can our experience of them. Large, flat flakes dissolve almost instantly on the tongue, while smaller, thicker granules dissolve much more slowly (notice I say “dissolve” and not “melt”, since the true melting point of salt is somewhere around 1500 degrees Fahrenheit). Sellers of flake salt can thus achieve a variety of effects by selecting for flakes of different sizes.
But again, this “variety” of flavor only holds so long as the flakes don’t dissolve, which is why an expensive salt like fleur de sel should only be used as a condiment. Add gourmet salt to anything that’s more than a little wet and/or warm, and your $20-a-pound luxury item becomes plain old NaCl again. And that puts us right back to where we started.
UPDATE: Reader Bronwyn from New Zealand adds:
Modern AMERICAN table salt manufacturers achieve the right conditions by using a purified brine (pumped out of flooded underground salt deposits) which is then evaporated in a special vacuum chamber. We don’t have any salt mines, so ours is all “solar”. See http://www.teara.govt.nz/en/salt/3. It’s something I always look out for when I’m flying over; the different shades of pink in the ponds are really pretty.