The salt we consume comes from two main sources – seawater, and salt deposits in the earth, left behind by ancient seas and lakes that have dried up. As a kid, if you’ve ever had an accidental gulp of seawater from swimming in the sea, you’ll remember that it tastes preeety salty. In reality though, the concentration of salt in seawater is on average just 3.5%, meaning for every 100 mL of seawater, there’s actually only 3.5 grams of salt in it. Just goes to show how potent salt is, eh!
To extract this 3.5% of salt, seawater is traditionally pumped into large ponds and left to dry out under the sun. There are still many places that processes salt in this manner, like around the Dead Sea in Israel and Jordan, and in Guerande where fleur de sel de Guerande comes from. But in places where this wouldn’t be feasible (mostly due to frigid temperatures and/or frequent rainfall), salt is often obtained through more industrial means, where salt water is first boiled vigorously, then the salt is forcefully crytallised out of the water, then centrifuged to dry it out. The whole process takes just a few hours.
If you remember from your secondary/middle school chemistry classes, the chemical formula for salt is sodium chloride (NaCl). This chemical composition allows it to form a lattice structure. So if you put regular fine table salt under a microscope, you should be able to see that the little salt crystals are in fact near-perfect little cubes. These are usually very pure salt manufactured under highly-controlled factory environments.
On the flipside, I’m sure we've all encountered those expensvie salt flakes that comes in different shapes and sizes. From ones that look like little Egyptian pyramids (characteristic of Maldon salt), to snowflake-like dendrictic salt, to little Machu Picchu lookalikes, salt flakes can take on vastly different structures. The shape of the salt crystals depends on several factors - the evaporation rate, the processing method, the composition of the trace amount of minerals found in it etc. etc.. At Jacobsen Salt Co. in the US, for instance, seawater is collected and boiled slightly to increase its salinity and remove impurities, and then fed into large open vats to slowly evaporate over 3-4 days. In this case, the salt is left to naturally form on top of the water, allowing them to coalesce into larger, pyramidal flakes, as opposed to the rapidly formed, tiny, cubical table salt.