The Etheria Crystal has many different skins that you can choose. You can change the crystal skin in the main menu screen where you pick your character to play.
These different skins are obtained by completing achievements in game and are as follows:
Classic : the skin you start with.
Crystalline : Unlocked by completing "Good Student".
Geode : Unlocked by completing "And This Is My Weapon".
Mystical : Unlocked by "Unknown".
Cell : Unlocked by "Veteran".
Unknown : Unlocked by completing "Brute Force".
Runic : Unlocked by "Unknown".
Landscaped : Unlocked by "Unknown".
Unknown : Unlocked by completing "Defender of Etheria".
Eastern : Unlocked by "A Taste of Victory".
Unknown : Unlocked by completing "A Challenger Approaches".
Unknown : Unlocked by completing "A Matter of Perspective".
Goblet : Unlocked by completing "Survivalist".
Unknown : Unlocked by completing "Iron Man".
Unknown : Unlocked by completing "Defense is the best Offense".
Book of Souls : Unlocked by completing "Obedience Training".
Unknown : Unlocked by completing "Perfectionist".
Unknown : Unlocked by completing "Daredevil".
Unknown : Unlocked by completing "Master Banker".
Unknown : Unlocked by completing "Team Effort".
Unknown : Unlocked by completing "Mastermind".
Unknown : Unlocked by completing "Cath 'em All".
Unknown : Unlocked by completing "Dungeon Defender".
Unknown : Unlocked by completing "Group Hug".
Unknown : Unlocked by completing "O' Mighty Smiter".
Unknown : Unlocked by completing "Legendary Defender".
