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If the holder uses a damage-dealing Dark-type move, then the holder consumes this item and the power of that move is increased by 50%. One-time use.
In single battles, Dark Gem is not typically a useful item, unfortunately. While the one-time boost to Dark-type attacks is nice, most Pokemon using Dark-type attacks would rather have the more reliable power of Life Orb or Choice items. Even so, some interesting strategies can be pulled off - one interesting play is to have a Pokemon with Dark Gem and Thief, such that the Pokemon would be able to get off a boosted Thief before stealing the opponent's item, as Dark Gem is used up before the attack hits. Of course, the downside is that you would have to use Thief in the first place, which uses up one of your valuable moveslots.
Typically Gems are more valuable in the VGC and similar environments, and Dark Gem is often used here to boost the power of Sucker Punch from Bisharp or Hitmontop, as well as upping the power of STAB moves for those that rely primarily on Dark-type attacks to deal damage.
Occur randomly in Dust Clouds.