I looked through the entire help and support section and I cannot find any explanation of a symbol that appears on some treasure cards.
I see this on treasure cards in my deck but not at treasure card dealers.
The symbol looks like a pointed wizard hat with a chin strap. It is located on the left between the amount of pips the card will use and the percentage of a successful cast.
That symbol is to indicate cards that have been earned through opening Treasure Card Packs. They typically have increased effects ( 95% shields, increased damage/accuracy).