A Crystal seed is what's left over after a Crystal bow or a Crystal shield is used up. Bows and shields can be purchased new from Islwyn without needing a crystal seed, or you can take your seed and bring it to Ilfeen and pay to have her sing to it to make it back into a bow or shield. You pay full price the first time but it lowers in price until the fifth time when you are at the bottom price. The bottom price for bows are 180 000 coins and for shields are 150 000 coins. Also if you lose your crystal seed, you can buy one from other players.
Upon completing Roving Elves quest you receive a partially charged (2/10) crystal bow or crystal shield.