The rarity seems to be shown by the color of the symbol inside of the "gem" on the cards. For example, if you click on "common" rarity, all the water droplets in the gems are grey. If you click on "rare" rarity, all the water droplets in the gems are blue.

The corner colors are gold for "life" element and grey for "death" element. Fire, water, and earth elements all have the same corner colors.

This is my reverse-engineering of the cards. I'm not involved in designing them in any way.

Thank you for opening up the booster packs. I was wondering what was in them.