A crystal seed used to be what was left over after a crystal weapon's charges were used up prior to the release of Prifddinas. Players can attune the crystal seeds to a crystal weapon seed.
Crystal seeds were obtainable by buying from players, buying new crystal equipment at a minimum price of 750,000 coins (crystal shield) and 900,000 coins (crystal bow) and then using them until they decayed into a seed or by getting them from a crystal geode obtained while chopping down a crystal tree.