A gem is a stones that is used to make jewelry. A crystal is apure substance that has molecules arranged in "a regular geometric pattern". A gem can also be a crystal, whereas a crystal can not be a gem.
Crystals are pure substances that have molecules arranged in a regular geometric pattern. A gem can be a crystal whereas a crystal will not always be a gem. Gems can have mineral bases such as ruby or diamond and an organic base such as amber. - - Hope I helped :)
