The Plague Gem is a gem added by the ScourgeCraft mod. They may be optained by killing Plague Archers or Plague Warriors.
Plague gems are used to craft Plague armor and Plague swords of varying tiers. They also are used to craft Plague Orbs which are used to the same end.
Each of the equipments that are created by Plague Gems have different effects, with new or improved effects added with each additional tier.