The Gem is a pick up that was removed. Before in a previous update, there was a collectable called a gem that awarded the player 1000 points. They were later removed and replaced with Coins. They were a purple-blue color and had a diamond shape.
Gems have been re added to the game but are now light blue or cyan coloured. They're now used as money along with coins.This is bad news for us as this is now a premium currecny that is near impossible to earn so say goodbye to super weapons like the heavy shotgun and avalance as now coins are useless and in a few update it is possible that you can buy gem weapons with coins so all we have to do is wait..........
Location
They were located in the same place where the Coins are in Campaign levels.