Grassium Z is an item in the “Z-Crystals” category (Key Items).
In-game descriptions
| Sun / Moon · Ultra Sun / Ultra Moon · Let’s Go, Pikachu! / Let’s Go, Eevee! · Sword / Shield | This is a crystallized form of Z-Power. It upgrades Grass-type moves to Z-Moves. |
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Where to find it
Not placed on the atlas maps (so far: Red/Blue and Generation 3).
Sources : PokeAPI (items, item_flavor_text, machines) · atlas · pokeemerald species_info.h