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If the holder uses a damage-dealing Grass-type move, then the holder consumes this item and the power of that move is increased by 50%. One-time use.
In single battles, Grass Gem is not typically a useful item, unfortunately. While the one-time boost to Grass-type attacks is nice, most Pokemon using Grass-type attacks would rather have the more reliable power of Life Orb or Choice items. It can potentially be used by Unburden Sceptile in order to activate its ability, but this strategy only works once, and is usually quite unreliable.
Typically Gems are more valuable in the VGC and similar environments, and Grass Gem is often used here to boost the power of Grass-type attacks such as Leaf Storm from Pokemon such as Rotom-C or Roserade. 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.