I've looked and looked but have found nothing that gives a key to the card border colors and decorations. I'm assuming they have to do with the rarity of the card. I've noticed:
pink (no frills)
light blue (sort of fancy)
gold (pretty fancy)
grey (pretty fancy too) - scenario reward
Can anyone help me out here, or point me in the right direction?
You list the cards in increasing rarity when found in treasure chests.
The copper border represents the Uncommon cards and these are generally ported-in cards from the Class Decks of the Pathfinder Guild. Some exceptions can be found (Fortified Leather Armor and two others, I think)
The blue bordered cards are Rare cards. Generally very good boons
The pink / violet (I am a guy, so no definite colour discernment ) are so-called Epic cards, which are much rarer, but sometimes very interesting. "Favor of <insert_god_name>" are my favourite ones.
The fancy gold border signifies extremely rare Legendary cards. Hard to get and usually very strong. Apart from Elemental Body (which I opened 2× + one as a bonus), I have only very few of these.
The fancy border without any significant colour (I think of it as gray) represents cards that you obtain as a reward for completing one whole adventure (3-5 scenarios) on Legendary difficulty. One exception (in the Android version, at least) is Guide to the Beyond, which incorrectly has the purple border (Epic).
There was some talk about limits of numbers of specific cards that you can own. That is, e.g. no more than X Elemental Body cards in your Vault (for me, it shows 2 instead of the 3 I received / opened). It is higher for less rare cards, as I have 5-6 of one uncommon card (don't know which) and I think 5 rare Shortbows +1.