Difference between GEM and CRYSTAL
Our body has layers of Aura around us whose intensity is decided by the frequency of the major seven energy centers [Chakras] namely, 
Muladhar [Root Chakra], Swadhistan [Sacral], Manipura [Solar Plexus], Anahat [Heart], Vishuddha [Throat], Adnya [Third Eye] and Sahaskar [Crown] Chakras. Each energy center is associated with mental, physiological, physical and health related parameters. 
Kidney to work normally at say x frequency if working on -x frequency starts creating issues. This can happen by birth or at times due to the improper lifestyle. Normally, chakras are moving in clockwise direction. If Chakra is revolving in –x then the energy center is moving in anticlockwise direction. If rotating with additional speed then normal say +x then too at times it may create a problem. 
For example if Muladhar chakra at –x frequency starts showing financial instability, constipation, taking wrong decision at times and such issues in their life. If it goes say -4x then probably constipation leads to piles and thoughts of suicide comes with extra stress issues. If say +4x, the person is too much stable and gets involved in drinking, bad habits, spending money in wrong deeds and related symptoms. So, basically our Chakras should be balanced in order to have healthy mind body and soul. 
Crystal are used to balance energy centers. For example from –x to x frequency. But GEMS have to power to increase from –x to +x frequency. But sometimes Its dangerous to increase upto +x frequency which increase the problem. But at times its very beneficial and can act as important milestone to enhance ones life. 
Crystals have no side effects. But there are more than 100 crystals. Which one would be beneficial for you can be checked through AURA SCANNING or a good Reiki Healer or Crystology expert.
Hence it is strongly recommended that you should consult a good astrologer to identify your gem or crystal.