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If the holder uses a damage-dealing Rock-type move, then the holder consumes this item and the power of that move is increased by 50%. One-time use.
In single battles, Rock Gem is not typically a useful item, unfortunately. While the one-time boost to Rock-type attacks is nice, most Pokemon using Rock-type attacks would rather have the more reliable power of Life Orb or Choice items. A very notable exception, however, is Substitute + Swords Dance Terrakion, who requires the power from Rock Gem to enable its STAB Stone Edge to OHKO its most dangerous counters, such as Gliscor and Slowbro, after a boost, as Choice items cannot be used for obvious reasons, and Life Orb doesn't cut it for power.
Typically Gems are more valuable in the VGC and similar environments, and Rock Gem is often used here to boost the power of Rock-type attacks such as Rock Slide from Pokemon such as Kabutops and Gigalith, which is useful thanks to Rock Slide's spread-damage effect. 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.