Sea salt is NOT pure NaCl; that's the whole point of eating sea salt! It has significant quantities of other minerals, such as calcium chloride and potassium sulphate. That's why sea salt tastes so different from regular table salt.
The reason people buy sea salt (made by evaporating seawater) or table salt (made by mining deposits left from prehistoric seas), is because no one produces these things artificially. There's no good economic reason to do so when you can just dig it up or dry out some seawater: making it artificially would be more expensive.
The same economics don't work for diamonds, for various reasons.
