Metroid Database Bestiary

Territorial cold-weather scavenger. The shell of a Crystallite reflects Beam weapons, and can only be cracked by a concussive blast. They hang upside down in an ice cave during their larval stage. Moisture runs off its body and forms the hard ice shell, which the Crystallite retains for the rest of its life.

You must destroy their hard shell with a Missile if you don't have the Plasma Beam. Crystallites are fairly docile creatures and don't go out of their way to attack you, so they're something you can easily avoid.