You are correct that copper sulfate crystals are a type of salt. A salt is defined as any chemical that is made up of a combination of positively charged and negatively charged ions. The salt is held together by the attraction between the positive and negative charges. Copper sulfate is a combination of positively charged copper ions and negatively charged sulfate ions, so it is a salt.

All kinds of crystals are useful and interesting to study, including protein crystals AND salt crystals! Salt crystals may seem common since we use them every day in table salt on food, but scientists are currently studying how salts crystallize, and the effects that crystallization can have in many different applications. For example, when monuments and artwork are left exposed to the environment, salt can crystallize on them. When this crystallization occurs, the growing crystals can exert pressure on the artwork, and potentially damage it. Therefore, scientists are researching ways to prevent salt from crystallizing on surfaces to protect historic structures from damage.