Unupgraded versions of shards can be won on the spin game if you defeat an opponent in the arena or when you win in a PVP duel. Upgraded versions can be won only in arena special spins in Gold (3 wins) or Platinum (2 wins) Leagues or 5 consecutive wins in Bronze and 4 in Silver.
There are some weapons, which don't belong to any element and are available in the Bazaar. But each element has a specific rare weapon. You can choose one of these when you complete Quest 7. Otherwise, you can win them on the spin game if you win against a Level 3 or stronger opponent, or if you are lucky enough when visiting the Oracle.
Nymphs can be considered as ultra-rare cards as they cannot be won on the spin game at all. If you defeat a Top50/PVP opponent whose deck has a nymph, then you have chance to win a Relic instead. Nymph cards can be won only from the Oracle. Similarly to rare weapons, each element has a nymph.
When you start to play Elements, first of all you have to pick up an element. By this choice you will get a mark card automatically (which can be changed by metamorphosis, in "Your Deck" menu). This card cannot be sold and is not part of your deck because it will be placed into the mark slot immediately when you start a game.
By winning on tournaments you can choose further element cards as a reward. And since this is the only way to obtain or upgrade mark cards, they are even more rare cards than Nymphs. If your opponent has additional mark cards you can only win a Relic on the spin game.
If you put mark cards into your deck they will act as pillars. If you play an unupgraded mark card that matches your mark, then it will be stacked in your mark slot (therefore it will not able to be targeted), while the others (along with upgraded mark cards) will be played out as regular permanent cards.
The remains of what was once a valued creature or artifact. Probably still worth a few coins.
The unupgraded version of this card can be won by the card spin from a Top 50 enemy deck if the deck contains special cards (nymphs or additional element mark cards). The only use of this card is selling it in the bazaar.