A gemstone is a mineral or crystal that can be cut and polished for use as a gem. (Also, gemstones have financial value based on rarity.)
A psychic can use both crystals and gemstones as tuning forks or grounding for certain types of psychic energies. From a psychic's standpoint there are only slight differences between crystals and gemstones:
A crystal, in its untouched form, is a symbol of nature and natural psychic energies. It is most used for impression, that is, to focus on and be influenced by the clarity of those energies.
A cut and polished gemstone, ready for use in ornaments or jewelry, symbolize the connection of humans and nature. Gemstones are used for expression, that is, to enhance, align or change the appearance of an item or person.