Rock salt is not widely known as a culinary salt, but there are a few areas where it can be very useful. The size of its grains and the fact that it does not dissolve as easily as more finely textured salts make it the best option in several cases. The downside is that you may have a hard time finding food-grade rock salt in anything smaller than a 40 lb bag unless you shop online. If you need it immediately, you will have to opt for a replacement. Consider the rock salt substitutes below.
Your best bet: Kosher salt
Kosher salt is the traditional salt used in the Jewish ceremonial preparation of meat. It is used to draw the blood out to render the meat kosher and was originally called koshering salt because of this. Kosher salt has the clean flavor of salt without the bitterness that some people associate with iodized salt. It also has larger crystals than table salt, which is the characteristic that makes it an excellent rock salt substitute. Another important similarity is the fact that kosher salt is free of additives like iodine or anti-caking agents, just like rock salt.
The large crystals keep kosher salt from dissolving quickly and allow it to be used in some of rock salt’s culinary applications, such as for making salt crusts. It can also be used to make ice cream, which is the most common way that rock salt is used in cooking.
When using kosher salt in place of rock salt, start with half as much as the recipe requires and add more if necessary. The smaller crystals mean that you can fit more into a cup when compared to the chunky crystals of rock salt.
A decent second choice: Himalayan pink salt
The Khewra Salt Mine in Pakistan is the source of Himalayan pink salt. This large-grained salt is mined just like rock salt, but contains traces of minerals that give it a pink color. The crystals are typically large in the most common versions of this salt. They are similar in size to the crystals of kosher salt. Extra coarse Himalayan pink salt is available from some retailers. The grains are closer in size to the grains of rock salt. As with kosher salt, the primary flavor is what you would expect from any salt.
Extra coarse Himalayan pink salt can be used as a 1:1 substitute for rock salt, but keep in mind that it is usually much more expensive.
In a pinch: Maldon sea salt
Unlike rock salt, Maldon sea salt is harvested from seawater. Rock salt is mined from salt deposits in the Earth. Maldon sea salt comes from Essex in the UK and has larger crystals than most other sea salts. The result is a sea salt with a similar appearance to kosher salt and that can work in many of the dishes that require rock salt.
Note that Maldon sea salt is prized as a finishing salt and is meant to be used for that purpose. As such, it is much more expensive than rock salt.
Use about half the amount of Maldon sea salt that your recipe requires for rock salt and increase if necessary.
Other alternatives
Table salt is less expensive than the other rock salt alternatives listed above and while it has iodine and other additives, it is still very similar to rock salt. The crystals are much finer, which means that you will need to use much less of it than the amount that your recipe requires for rock salt.