Crystal Nodules first appear in Pikmin 3 and appear mainly in the form of walls or by withholding fruit from a captain. They come in many different shapes and sizes and are often considered one of the most prominent obstacles of the game.
Rock Pikmin
Rock Pikmin are the only types of Pikmin able to smash through the crystal walls and nodules (as they have a "rock hard body"). Without the rock Pikmin masses of fruit would be unobtainable.
It should also be noted that the Rock Pikmin have the advantage over the Skutterchucks as they are able to smash the crystal rock they wield, thus rendering them defenseless.
Enemies & Bosses
As well as the Skutterchucks, there are other enemies who use the crystal for protection: the Armored Mawdad,Quaggled Mireclops and the Calcified Crushblat.
Armored Mawdad: This creature is the first boss in the game and can be found in a hollow tree in the Garden of Hope. It will use its crystal exoskeleton to defend against Pikmin. Rock Pikmin are required to crush the casing and reveal its weak skin. The Mawdad's tail is its weak spot for both the crystal and the flesh and so should be attacked there for an efficient defeat.
Quaggled Mireclops: This is the penultimate boss in the game and uses a crystal to protect its head. When first discovered after the escape of Louie it appears completely placid, until the player heads over and attacks the beast's crystal casing.
Calcified Crushblat: resting peacefully in a well concealed pond (in the Tropical Wilds), the Calcified Crushblat will threaten anyone who comes near it. It is only when a squad of blue Pikmin have been turned to rocks will it be vulnerable. It threatens to crush any who come too near and so for that reason it is best to keep any remaining blue Pikmin to the side.