Logia fruits make up around 10 of the 36 fruits in the game, including Gas, Flame, Smoke, Magma, Ice, Light, Darkness, Sand, Rumble and Snow and are useful to have.
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What it looks like to be hit by a Beast Pirate with a logia fruit. (Magma)
Logia fruits make you invincible to basic sword and combat attacks without using Busoshoku Haki. Logia is not invincible to PvP Devil Fruit attacks.
When you are hit with a logia fruit, a particles related to your fruit show up around your body and you take no damage. (i.e. Flame = Flame particles, Snow = Snowflake particles, Sand = Sand particles, etc.)