Many minerals and compounds containing sulfur have an unpleasant odor. Strike a match. The smell is sulfur. Sulfuric acid is used to clean pipes by plumbers. You will see it in most products used to unclog drains. It is a dangerous product and the fumes can be deadly. Some crystals are stable and will not have an odor, best to ask the shop you buy from how to display, preserve your specimen to avoid any unpleasant odor or decay.