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If the holder uses a damage-dealing Poison-type move, then the holder consumes this item and the power of that move is increased by 50%. One-time use.
In single battles, Poison Gem is not typically a useful item, unfortunately. While the one-time boost to Poison-type attacks is nice, most Pokemon using Poison-type attacks would rather have the more reliable power of Life Orb or Choice items.
Typically Gems are more valuable in the VGC and similar environments, and Poison Gem is rarely used here to boost the power of Poison-type attacks; however, given the poor coverage of most Poison-type attacks, it is possibly the least used of all the Gems, particularly as all the viable Poison-types have better STAB options they could be using. For those that do not have a choice, however, the one-time power boost can come in very useful in such a fast-paced metagame, especially as it comes without any drawbacks.
Occur randomly in Dust Clouds.