The crystal armour seed is an elven crystal that can be turned into crystal armour. Completing The Gauntlet will yield a chance of gaining these seeds and combining them with crystal shards can be used to create the armour pieces.
Players can also exchange the armour seed for 250 crystal shards by speaking to Amrod in Prifddinas' Hefin Inn.
Players who do not meet the Crafting and Smithing requirements for each piece of gear can talk to Conwenna or Reese to sing the armour pieces for them for an additional 60 crystal shards per piece.
For an exhaustive list of all known sources for this item, see here