The Adamant Turtle is an enemy from Final Fantasy Mystic Quest. The turtle attacks with fire, but is weak to Axe-type attacks. As Reuben's weapon is an Axe-type weapon, this means the Adamant Turtle can easily be slain by him along with the player's own Axe.
Adamantite and similar words including Adamant and Adamantine are used to refer to any especially hard substance, whether composed of diamond, some other gemstone, or some type of metal.
In the German release, the enemy is known as "Maden Fresser" which means "grub eater".