The Crystals, Accessories and Energonicons in the game can have different rarities. Each rarity has it's own colour that is used all around in the game.
Common - white
Uncommon - blue
Rare - green
Epic - purple
Legendary - gold
The rarity of a Crystal can be viewed by clicking on the Crystal in your Storage.
The rarity of an Accessory can be seen by the colour of the outline in the icon.
The rarity of an Energonicon can be seen by the colour of the outline in the icon. In Pig Lab you can also view all your unlocked Energonicons with the rarity shown at the top of the page.