Gems are a valuable secondary currency in Spartan Warfare.
You gain gems by doing scouting turns and challenges.
Gems can be used to refil your WILL bar or ENERGY bar. This can be done at the gem temple (explore>base exchange>gem temple).
Gems are sometimes called "crystals," this is a minor flaw that appears in content from earlier in the game, however there is no difference between the two.
Gems are also used to buy upgrades for a platoon, (ie. player capacity, garage size, armory size).
Gems can be bought and sold in the gem market (explore>base exchange>gem market), you can also buy gems with points [LINK!].