Salt is available in several forms with names that reflect the origin or intended use of the particular salt. Sea salt, for example, is from evaporated seawater, and rock salt is from underground deposits. Table salt is meant for household use. The push to reduce sodium in the food you eat has led people to search for substitutions that are lower in sodium than table salt. The different names given to other salts makes them sound like promising alternatives, but they are not.
Despite the different names, rock salt and table salt are the same thing, only in slightly different physical forms. Table salt is rock salt that's been finely ground. Table salt can be iodized, or have iodine added to it, but the sodium chloride content of both is the same. Colorado State University Extension reports that table salt is about 40 percent sodium and 60 percent chloride. Salt as a whole is sometimes called by its mineral name of halite.
Rock salt is either mined from underground deposits or from above-ground water sources that have since evaporated. According to the Salt Institute, the “room and pillar” method of mining is used for underground rock salt deposits; mining space is blasted out while leaving support pillars of unbroken salt. The salt is crushed down, packaged and sent to other companies. Those who manufacture table salt grind the rock salt down even further and refine it to remove other minerals or substances that might have gotten stuck in the salt bed as it formed.
Rock salt is used mainly in deicing formulas for roads in winter. It is also used in the salt/ice layer in ice cream makers. Table salt is meant for eating and cooking only -- no pickling or canning. Penn State Cooperative Extension says the additional anti-caking agents you usually find in table salt can cloud pickling brine, and the iodide can change the color of the foods inside the container.
Table salt is formed in such a way that much of the extra minerals or contaminants that were trapped in rock salt deposits are siphoned out, leaving mostly sodium chloride plus whatever additives the manufacturer decides to put in to ward off caking. This image of a natural salt deposit cut down and refined to the point that it is mostly sodium chloride could make table salt seem like it contains more sodium than other types of salt, but the University of Missouri notes that this difference is slight.
In addition to the basic ways of cutting salt out of your diet -- cooking from scratch, using spices instead of salt and not salting pasta water, for example -- follow Colorado State University Extension’s suggestion of blocking part of the salt shaker top, which will reduce the amount of salt able to fall out when you shake it. If flavor is the issue, rather than just the quantity, reduce what you use gradually and get used to a small reduction before cutting out more.