Monolith

Monolith (石頭) is one of Lexaeus's weapons in Kingdom Hearts 358/2 Days. It is a moai.

Design
The Monolith is seemingly made entirely out of grey rock. Its “blade” takes the form of a moai in profile, while its counterweight is a flat section of rock that juts out from the back of the moai’s head.

A monolith is a large block of stone used for sculpture or architecture. It can also refer to anything massive and unmovable. "Rockhead" clearly refers to the weapon's appearance, that of a stone head.

Since the description for the Mystery Gear states that it brings out the user's personality, this may be a reference to the fact Lexaeus tends to be "stone-faced".