This item is autokeep. This means that if you die with it, you will always keep it.
To obtain a rock-shell plate, players can either bring 3 dagannoth hides, 1 rock-shell shard, and 10,000 coins to Skulgrimen in Rellekka, or receive it as a drop from Dagannoth Rex. The plate can be considered as a second alternative to the rune platebody, as completing the Dragon Slayer quest isn't a requirement to wear the rock-shell plate.
The Rock-shell plate is a piece of melee armour that occupies the body slot You must have 40 defence to wear the rock-shell plate. Obtaining a rock shell plate is rare drop from rock crabs.