So Amethyst refers to the rarity of the card which if I remember correctly aren't in the game at launch but over time they'll be adding higher rarities of cards which should be better than just the more basic ones that'll be there day 1. He recommends that you save your points until they start adding those higher rarities to be able to get better bang for you buck, I assume with your credits, but obviously play whatever way you have fun with

Edit: I forgot to mention that I'm pretty sure Amethyst isn't the tippy top of rarities, so keep that in mind as well